cadillac heated driver outside mirror discolored

happened after he bought from dealer and dealer replaced whole assembly for I think he said 800 dollars.I know you can replace just the glass seperately.......my question is what is causing the heated mirror to discolor and get all funky looking and if there is a problem with the module driver door controlling the current or a problem with the heating element under the mirror and how to test or a common issue.anyone have any ideas.I don't want to just replace glass only to have it happen again.thanks in advance.

That's a good question, I've seen plenty of heated mirrors turn brown or black, but figured it was just a shelf life issue. I'm curious to see if anyone knows why they do that, or why a replacement would turn prematurely.

Also, I've had little success changing the glass and usually opt to change the assembly.
Maybe I've just been using the wrong hammer...

I'm kinda wondering if this is a defective part problem, too. Shelf life, like Noah said. But this is what I found for wiring:



I'd probably start by watching voltage on the supply wire to the drivers mirror, and watch it's behavior while you're operating the defrost switch. If I remember right, these systems also have a timer involved, so it shuts the heater off after a certain amount of time, regardless of switch position. Watching the behavior of the passenger mirror may give you some clues.

Not sure if you have an amp probe, but comparing current measurements between the two would also be a good place to start.