Rear defogger

Can you tell me how to wire in a rear defogger in a truck that didn’t have it from the factory? I’ve replaced the rear glass with one that has a defog grid and contacts. How to wire a switch, relay, etc?

To do this, you’re going to need a relay who’s contacts are rated for 40 amps and a relay socket. You’ll need to install a dedictated 40 amp fuse from the battery. You’ll need some 10 gage wire to handle the current from the battery to the defogger, and some 16 gage wire for the control voltage to the relay. And you’ll need a switch that lights up when voltage passes through it so the defogger isn’t left on when the vehicle is off.

We’ll start with the primary or control voltage for the relay. From the battery using the ten gage wire connect one end of the fuse holder. From the other end of the fuse holder run the 16 gage wire to one side of the switch. From the other side of the switch run a 16 gage wire to the relay socket for one of terminals for the primary or coil side of the relay. From the relay socket terminal for the other side of the primary or coil run a wire to ground.

The secondary voltage for the relay. This will all be 10 gage wire. From the 40 amp fuse holder, run a wire to the relay socket for the terminal for one side of the relay contacts. From the relay socket terminal for the other side of the relay contacts, run a wire to one of the rear window defogger connections. From the other rear window defogger connection run a wire to ground.

Clear as mud?


What year is this truck?

A question to all others: These defrost switches generally or always have a timer function to shut themselves off after a period. Does anyone know why? I always guessed to save the battery since it is a large power user.

Saving the battery and alternator is most likely answer. I think there were cases of the glass breaking on these windows in the early days due to excessive heat and cold ambient air temps.

Thanks, I think I can work that out. It might be easier if I got a factory installed set up from a salvage yard. Alas, 1989 trucks are in short supply in my area! LOL
My biggest confusion was in whether to run the second contact from the defogger directly to ground or not. Thanks for the answer.

A salvage yard would have been my suggestion. Except for the panel switch, parts and wiring for the rear defroster shouldn’t vary all that much from vehicle to vehicle. Maybe you don’t need an exact model year match. Talk to the nice folks at the salvage yard. Maybe they’ll have a thought or two.

I always assumed the purpose of the timer is to prevent overheating the coils and cracking the window. Saving electricity seems only a secondary reason.