I heard that heated mirror is standard equipment on all 04 WRX, but it is not available for STi. It is controlled by rear defroster...
Is this true? If so, can they be interchanged?

THanks

It is part of the 04 premium package. I don't think that option is available for the STI since the spoiler is included in the package and the STI comes with a bigger spoiler. Also in the package is the sunroof, heated seats, and heated winsheild wiper strip. And yes it is controlled by the defroster. When I turn it on I hear a click coming from my mirror.

I have seen aftermarket pads that can be put on the backside of the mirrors and then wired to a switch. They are held on by tape that is on the pad.