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I added 05 mirrors to my e99. For the heat switch, I used the defrost switch out of a 97 explorer. It has the timer built in so you just need power in, ground and power out. You will have extra wires but the only other one that you might use will be for illumination.
Correction....Thats a 94 Explorer... Only wire I had left over was illumination. Now that I see this I also remember just tying the top two wires together to switched 12V so it can only run with the ignition on.
The early years, prior to '08 (I believe) were constantly on with key on. I think '08 was the first year they put a seperate switch for the mirror heat on the super dutys. I wired up a seperate switch for the heat on the '08 mirrors I put on my truck.