I have a set of OEM Schefenacker power, heated, signal TTT mirrors and I am having a problems with my passenger side one. The problem is that when I turn on the defrost switch for the mirrors the passenger side mirror lights go out completely and it's only the passenger side because the drivers side works fine.The mirror still has power because I can still move the glass. Yesterday I took my truck to the dealer to ensure that there wasn't any problems with the truck wiring which there wasn't. I've already switched out 2 mirrors and both of them did the same. I'm waiting for my third one to arrive. Has anyone had this problem. I think the problem is with the ground wires on the passenger side.

 

Sounds like a classic bad ground. The defroster and the light likely share the same ground wire. If that ground wire is open, the defroster will ground through the parking light circuit, taking out the ground path for the light.

To test it, clip the ground wire in the mirror and run a jumper to a solid chassis ground. If the light and defroster begin to work as they should, then you know exactly where the issues is.

Let us know what you find.