Problems with 1997 tbird rear defrost switch

My rear defroster on my 1997 Ford Thunderbird will not turn on. My mechanic said that the rear defroster gets power and can turn on when he test it directly. I have bought two used rear defroster switches on because my mechanic. said that the rear defrost switch is not working. The mechanic tried installing both switches and neither will light up or turn on the rear defroster. I am unable to find a new rear defrost switch and at this point I can not think that both used switches I bought were bad. Does anyone know anything else to try to fix this problem? I can not understand how the rear defroster can get power and work when my mechanic test the power, but yet no switches work. any suggestions is greatly appreciated.

I have no idea what your mechanic means when he said he “Tested it directly”.
You should be able to test the switch.

It could also be a bad ground.

Switch has a relay, switch, light and led in it. There are 2 power sources for it. I would check the fuses if I were you.

Here is the schematic. 1 fuse is under the hood in the Power Distribution box. The other fuse is in the Primary Junction Block under the dash and to the left of the steering wheel.

The mechanic has some power device to temporary power defroster. If a fuse is bad, would it cause switch not to work

If a fuse for the circuit is open it wouldn’t matter if the switch worked or not, no power would go to the rear defroster heating element. I presume what your mechanic has done is test the element, and that’s ok. ?but it won’t work if it gets no power. It’s like if you had a electric stove with every function working, but the homes’ circuit breaker had tripped off. You’d get no joy w/that configuration.

The way an auto-electric expert would test, after checking that the element worked, he’d then check if voltage was getting to the element. If not, he’d work backward from there toward the switch to find out what’s wrong.

Usually switches which only power relays seldom fail. The most likely failure in high current circuits like is a burned connector, and after that, a relay on the fritz.

Your switches can be easily tested with an inexpensive DVM. They should have less than 0.1 ohm resistance when “on”. And infinite resistance when “off”.

Check with a local salvage yard (they have an interchange book) and see if the defroster switch was used on other Ford models. I’ve done this many times and you would be surprised at the number of interchangeable parts out there. In my mind…you have another problem and the defroster switch is probably ok. The defroster switch can also be checked very easily with the “ohm” setting on your multimeter.

thank you to everyone who has posted so far. If the switches test good, then what else should I ask my mechanic to look for?

Wiring, relays and fuses. Have him check the light on the defroster switch as well. If it doesn’t come on when the switch is activated…the defroster will not work. Good luck.

As the drawing shows, power should be on the yellow and the pink/orange wires going to the switch. The trouble is most likely due to one of the fuses for those two lines is blown out. This is a pretty simple circuit and should be easy to figure out where the trouble is at.