I don't have exact details but I can give you some ideas. You're going to have to determine which wires in the wiring harness to the door powers the defrost function. You're then going to have to tap into/rewire those wires so that they will be attached to the relocated mirrors. Depending on where you relocate them to, you may have to cut into the body. Hopefully someone else here will have better instructions. Make sure that you take a lot of pictures and post a "How-To".
Are you permanently moving the mirrors? Otherwise if you're relocating the mirrors just to take the doors off then you're probably not going to need to defrost the mirrors. In my opinion I think the doors look silly with the mirros removed and that black disc to hide the holes.
Why not just put the re-locate brackets on, move the mirrors in the spring, do doorless when you want, in the fall put the mirrors back on the doors, unless you plan on going doorless AND using the defrost function, in which case i think you have a bigger problem, nut warmers may be needed!