The heated mirrors on my 1999 Subaru Forester are controlled by the rear window defogger switch which is located in the dashboard console above the heater controls and to the right of the hazard flasher switch. I'm not sure whether the heated mirrors were optional on the early Foresters or not.
Anyway, the heated mirrors (and the rear window defogger) will stay on for about 15 minutes when you press the switch. You can turn them off before then if everything has defogged already. Also, the switch only works with the ignition on.