Heated Mirror Melting

[IdahoBob]

This is on the 2002 XC70, 160,000 miles. No significant electrical issues so far; then today, while washing off the road/snow gunk I noticed that the plastic fairing/housing around the passenger (right) side outside rear view mirror is crinkling up. The part against the door is getting some good orange peel texture and shrinking, and the very outside edge of the mirror housing is developing a patch of the same damage to the plastic. Looks very much like it's getting too hot.

Is there a thermostat in this thing somewhere that tells it to turn on and off, or is there something else I should be checking? If it's a part I need to check/replace/curse, where is it?

I appreciate the help.

IdahoBob
(Denver, Colo.)