Hi all,

My drivers side mirror was damaged recently and I replaced it was a 960 mirror that has the heating element. I thought maybe if a get Rear/mirror Defrost combos switch off a 760 it might work. So I went to the junkyard and sure enough I found a 760 with the combo switch. I removed my current 740 single unit switch and replaced it with the newly acquired switch. I also noticed the switch had 4 connectors as opposed to 2 on the old switch. I thought maybe I had something here. No luck. Rear defrosts works but not the drives side mirror. Is there some wiring that the 960 had and the 740's did not have to make this work. I hate having the side mirrors frozen on a cold morning as this being a safety issue. You think this would be an option on the 740's back in 92. I know it was on the 940's. What am I missing to make this work? Thanks for your advice.
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Regards, Chris

I installed heated mirrors on my 1987 745T. It was just a matter of installing the mirrors and changing the switch. The 700 series had the wiring for heated mirrors starting in the 1985 model year. For my son's 1984 760T, I had to replace the electrical connector at the defroster switch, and add the wiring into the doors. You car will have all the wiring for a plug and play installation. Check for voltage at the wiring connector inside the door. What model year of 760 did the replacement defroster switch come from?
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john

Good question. Unfortunately I didn't look to see what year that model was.
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Regards, Chris

The 1987 and later 740s with the fine wire rear window defroster use a different defroster switch for the higher power requirements. If you use an earlier switch you may toast it after some use. The wires to the heated mirror elements should be blue with a green tracer. Check for voltage at the switch. I have never had trouble with the connections at the heated mirror element, but I remember other posts reporting trouble with the connections at the mirror. You may need to remove the mirror and clean the connections.
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john

I looked at the wiring diagrams in my Mitchell CD. They show the 760 switch circuit for the rear window and mirror defrost to be the same as in the 960 and the 74'92 740. However if your 740 did not have the heated mirror option, it may not have the needed wiring for the mirrors. The wiring for the heated mirror option is as follows:
Switch pin 4 = Blu/Yel and goes to 12v through a fuse which is hot in Run/Acc
" " 2 = Yel/Red or Yel and goes to heated rear window
" " 1 = Blu/Grn or Blu and goes to mirrors
" " 3 = Blk and goes to chassis ground

You may have to run a wire for the heated mirror. Sometimes the manufacturer will have a wiring harness with all the wires for possible options included. Maybe your car has the needed wiring in place but not connected to the switch.

Hi Jim,

Thank you for this information. I reviewed what you mentioned above and I have all connectors present. It would look like I have all the proper connections. You mentioned, "Maybe your car has the needed wiring in place but not connected to the switch". What do you mean by this statement? Should I take the mirror off and verify the correct wiring is in place from what I see at the switch side? If so, then what? It would seem to me that I am all set. I also checked the fuse and that’s good since my rear defrost works well only the mirrors are not working. One other thing I noticed. The #10 fuse is a 30 mini. All the other fuses are the normal size. Seems funny.
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Regards, Chris

Do you have 12v at pin 1 of the mirror defrost switch with the key in the run position? You should have 12v for about 15 minutes. If you have 12v there then I'd check to see if you have 12v at the mirror. If you have 12v at the mirror, I'd check to see if the other connection to the mirror heater goes to chassis. I'd also check the mirror heater's resistance to see if there is continuity through it. I don't know what the heater's resistance should be.

What I meant about the wire harness was that it is sometimes cheaper for the manufacturer to have one harness made whic hcan be used for all options. For example, the wire for the tachometer in a 240 is there even when no tach is installed.