97 528i Heated Mirrors

I have a 97 582i and I have question regarding the power side mirrors (drivers side and passenger).

Up her in Canada the winters are pretty cold and the salt on the roads take a toll on the car. I try and take the car and have it washed regularly and have experienced a minor problem.

Aftre the car is washed I notice that ice forms on the side mirrors which does not melt.

Question: How do the headted mirrors work? Do they come on automatcially? Is there a switch to activate them? Do I need to set the front/rear defroster to "on" to get them to work?

Thanks in advance to all those who assist with a reply.