My '99 SR5 does NOT have heated mirrors. The spot for the fuse, which should be next to the 15A Rear Defog in the engine bay fuse box is empty. The wires might be in back, I don't know. The odds are they are there.
It would be a green/black from that fuse (10A) through a couple of connectors to each front kick panel and then through the door to each mirror, plus a white/black ground from mirror back to a ground point in each kick panel. In the fuse box, the feed to the MIR HTR fuse needs to be tied to output of the defog relay, as is the Defog fuse. So I would look for those colors in either kick panel and see if they go where they should go. If that looks good, open up the fuse box in the engine bay and see what is missing there.
The hardest part, of course, is from kick panel through the door harness to mirror. If heated mirror was not even an option in your year, then none of this probably is in your car.