The rear window defogger works fine but neither outside mirror heats up on my 1997 850 wagon.It is on the same switch with the same common 30amp fuse which is good. Any ideas? Thanks

 

Have you checked that the mirrors are plugged in..

I had a similar problem recently, just pull the mirrors of by holding the inner most area and pulling towards you.

check that the 2 wires are plugged in and making good contact.