Well I'm expecting to control my heated mirrors through the rear defrost circuitry as well. I put my meter on the green and purple wire in position 1 and ground and turned the rear defroster on and off a couple of times and verified that it will control the power to heat my rear view mirrors. It should work just like yours do.
I owned my truck for two years before I needed to defrost the side mirrors. I never thought to turn on the defroster because the rear window was under the eaves of the house and didn't have any frost on it. When I got to work I jumped on here and had to search how to turn on the side mirror heat. I would have never figured it out. I didn't even show up in the owners manual. Glad you figured it out.