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Rear Window Defogger Installation
for 1968-1971 Thunderbird and
1969-1971 Continental Mark III

This is an easy option to install on your T-bird or Mark!


Parts Needed

C8SZ-18527-A Rear Window Defogger Motor and Wheel Assembly
Motor Assembly Mounting Clips (x4)
Rear Defogger Control Switch
(click for switch part numbers, listed below)
Correct Instrument Panel Left Air Control Assembly
(not necessary for installation, but needed for 100% correct installation)
Flat Head and Phillips Head Screw Drivers
Hex Head Nut Driver
Several Hours
(this can be done solo, without any additional help)

Thunderbird Rear Window Defogger instrument panel controlThis is probably one of the easiest options you can install on a 1968-1971 Thunderbird or 1969-1971 Continental Mark III. Introduced as a new option for 1968 on the T-bird, the Rear Window Defogger was only available through 1971, after which the grid-type Electric Rear Window Defroster would be the sole choice. To install this option, you basically need to locate just two main parts: the instrument panel on/off switch and the defogger unit itself. The defogger is a one piece assembly, which takes in air through louvers in the rear package tray and then exhausts it through internal ducting onto the inside surface of the rear glass. It's mounted behind the rear seat, under the package tray.

A two speed fan controls the air velocity, which can be as high as 45 cfm on the high fan speed setting. Not only will it defog the rear glass, it also helps to melt light frost or ice, and circulates air in the rear seat area when maximum ventilation, air conditioning, or heating is needed. The low fan speed is almost inaudible, even to rear seat passengers, so normal conversation is possible inside the car with the defogger running. The high speed makes short work of defogging the rear window, so you won't need it on this speed for long, and since good air circulation throughout the car is essential for overall comfort regardless of whether you need heating or cooling, the rear defogger can be used for more than just clearing the rear window of fog.


To install, you'll need to remove your rear seat cushion and seat back, (as well as the quarter arm rests and center armrest or filler panel on Tudor Thunderbirds and Mark IIIs.) Follow the instructions in the Thunderbird Shop Manual (you need one if you don't have one). Next, remove the center section of the rear package tray by removing two screws on top of the panel and the screws below the front edge of the tray (concealed by the seat back when it's installed). If your car has the optional Rear Lamp Monitor, be very careful with the plastic fiber optic tubes, they may be brittle due to age! On Tudor Birds, you have to also remove the aluminum trim panels as well, which can be a tough job. (Just follow the instructions in the Shop Manual, but be careful they will dent easily.)

Once the rear package tray is out of they way, you can remove the sound absorption pads and insulation from around the center area of the package tray where the defogger motor will be located. You'll see a metal filler plate has been installed at the factory in all cars without the defogger option. It's now a simple process to unscrew a few screws, remove the plate from the hole, drop in the defogger unit, and secure it with 4 retaining clips. Twist each clip 1/4 turn to lock them in place. And yes, it's truly that easy.

Accessory Rear Window Defogger MotorThe defogger motor assembly itself (pictured at left) is a one piece plastic housing that consists of a balanced integral defogger motor and duct work. It has two wires attached to it. The ground wire is the one with the ring terminal on the end of it, and it grounds to the metal shelf under the rear package tray. The hole where this ground wire attaches is already there, to the right of the assembly as you're looking at it from inside the car. The other wire is the power lead with a male connector on it, and it plugs into Circuit 261—red/black stripe wire* with a clear female connector on it that's part of the main wiring harness in your luggage compartment. You'll find it coming out of the wiring harness just behind the back seat, with all of the other wires that run in that harness to the luggage compartment.

*Note: The color coding on the wiring diagrams may be wrong, most of the power feed wires we've seen are actually orange with black stripe, as pictured at left. At any rate, find the wire with the single female connector on it as pictured and you'll have the correct wire. It's the only one like it in the luggage compartment.

The only other thing you need is the instrument panel on/off control switch, which means you need to locate the correct part. There are several variations of this switch, so make sure you get the correct one. Here's your shopping list of the parts you'll need, including the part numbers:

- C8SZ-18578-B 1968-1969 Ford Thunderbird or Continental Mark III
Heater only or Manual SelectAire Conditioning with Rear Defogger
(3-speed heater blower switch, 2-speed rear defogger switch - total of two switches mounted on the bracket)

- C8SZ-18578-C 1968-1971 Thunderbird or Mark III
Automatic Climate Control SelectAire with Rear Defogger
(2-speed rear defogger switch only - one switch mounted on the bracket)

- DOSZ-18578-B 1970 Thunderbird (built before 5-1-70)
Heater only or Manual SelectAire Conditioning with Rear Defogger
(4-speed heater blower switch, 2-speed rear defogger switch - total of two switches mounted on the bracket)

- DOSZ-18578-E 1970-1971 Thunderbird (built from 5-1-70)
Heater only or Manual SelectAire Conditioning with Rear Defogger
(4-speed heater blower switch, 2-speed rear defogger switch - total of two switches mounted on the bracket)

- C8SZ-18527-A (MM-245) 1968-1971 Thunderbird and Continental Mark III
Rear Window Defogger Motor and Wheel Assembly - Includes Housing (Balanced)

Note: You will need the 4 mounting clips to mount the motor to the package tray, so retain the clips if you remove the motor from a parts car.


What's the difference in the switches? Remember that 1968 and 1969 Thunderbirds and Mark IIIs had a 3-speed blower switch for the heater/defroster and manual temperature control air conditioning, while the 1970 and 1971 T-birds and 1970 Continental Mark IIIs had a 4-speed blower switch for the heater/defroster and manual air conditioning. (Automatic Climate Control was standard on the 1971 Mark III, so there was no separate blower switch.) You can grind off the rear defogger switch if necessary and fabricate your current blower switch mounting bracket to accommodate the rear defogger switch, but it wouldn't hurt to find the correct one, just in case.

Remember the instrument panel left air control assembly should have lettering that designates "OFF REAR DEFOG ON" below the rear defogger switch. You don't have to have the correct panel for the defogger to work properly, but no car is correct without this designation. (Also remember: you need to get the correct panel for climate control if your car has that option, as it is supposed to have the AUTOMATIC CLIMATE CONTROL lettering at the top.) The bracket itself just screws in to the bottom area of the heat/air conditioning control head with 2 small hex head screws, which you can reuse on cars without climate control. If you have climate control, keep the screws from the donor car to mount the rear defogger switch bracket to the control head.

The wiring and plug for the control switch is part of the main dash wiring harness on every car, and may be taped up inside the harness itself in the area near where the switch mounts, or the plug might just be dangling behind the instrument panel. It won't be hard to spot, it has 4 wires going to it:

Circuit 257 - solid yellow (power feed to control switch)
Circuit 48 - solid black (ground - terminates in harness and cannot be checked for continuity)
Circuit 260 - violet (resistance wire for low speed motor operation)
Circuit 261 - red/black stripe (power lead to defogger motor; may be orange/black stripe)

Once you've located this plug, connected it to the switch, and installed the switch in the instrument panel, you're all set to go. With the key in the accessory or on position, the rear blower fan should operate.

Image: 1969 Continental Mark III Rear Window DefoggerInstrument Panel Left Air Control Assembly Removal and Installation

To remove the instrument panel left air control register assembly, disconnect the battery ground cable, then remove the headlight switch knob by pulling it to the full "on" position, then reach up under the instrument panel and locate the small knob release button on the bottom of the switch. Press it in and hold. While holding button, pull out headlight knob. Then remove the screws holding the air control assembly to the instrument panel, unscrew the headlight knob bezel, and carefully remove the air control assembly. You may want to protect the steering column to prevent scratches, or if you want to do it right, remove the lower inspection panel from below the steering column and remove the bolts holding the steering column to the instrument panel. This allows the column to drop down slightly, which provides additional clearance to remove and install the air control assemblies on either side of the steering column.

When installing the headlight knob, you may need to press the button in while inserting the knob and shaft. Make sure the knob is locked in place and cannot be pulled out of the switch. If it can, it isn't installed properly. Sometimes it can be a bit testy, but it's important to make sure the knob can't be pulled out when turning on the lights!


Reverse the removal procedures above to reassemble. Be sure to check the blower for proper operation before putting everything back together. Make sure the vents in the rear package tray aren't blocked by padding when installing that part.

Allow several hours for installation, the majority of which will be spent removing and installing the rear seat and package tray. No additional help is needed for this installation. You will really enjoy having this option on your car. It can truly be used year round to provide increased air flow in the back seat area during times when maximum air conditioning is needed, as well as having a clear view to the rear in humid areas of the country.

Finding the Parts

Many of these items are available periodically on eBay. We suggest watching our Classic Car Parts Auction feeds for them:

1968 Thunderbird Parts | 1969 Thunderbird Parts | 1970 Thunderbird Parts | 1971 Thunderbird Parts

1968 Continental Mark III Parts* | 1969 Continental Mark III Parts | 1970 Continental Mark III Parts | 1971 Continental Mark III Parts

*We realize Lincoln didn't build a 1968 Mark III, they were all 1969 models built during the 1968 model year. However, people list parts for sale as 1968 parts due to the way they're numbered. So, we've included a page searching for those parts.