My left hand mirror dropped out recently and I replaced it with a junker. In the replacement I found a wire to the mirror. What is this for? Heat?

Yes. The outside side-view mirrors (both sides) have little heating elements behind the glass that automatically go on (below a certain temperature -- 36 deg F, I think, but it's something close) to remove sleet and snow that might otherwise collect on it. Neat?

Ken, thanks for the feedback. I could have tried to read the schematics, but I figured some old hands already knew. On a slightly different vein, is there any replacement for the mirror glass without having to buy the whole silly thing? It is rather expensive mirror bracket.