Food for thought:
The early MLs (and other models) now have issues where the electrolytic fluid between the glass panes starts to leak out. This is the stuff that reacts to light and allows the "dimming" feature to work. Some folks had replacements under goodwill but for the most part, it's out-of pocket. My ML has this problem.
A new ML mirror at the dealer will set you back about $670!
My CLK has 'em on both outside mirrors too (at $350/ea. for just the glass replacements...hope they hold up for awhile!).
Ironically, the ML "non-dimming" versions go for about $66. I guess it is a design issue with the install difficulty, as stories at the shop abound with shattered ML windshields from improper removals of the affected units!
Some later mirrors (like on the CLK) also have the garage opener controls integrated into the mirror bezel. That makes for a more complex harness to work out.
I'm going to just clip one of those panoramic mirrors over my failed mirror...
In your case, put a test lead on each wire and shine a flashlight onto the mirror and see if the resistance changes.