From what I've read you are correct, they are activated with the rear defrost. Should be fairly easy to wire in then. I thought they were switched on with a control on the driver door just under the mirror directional buttons. Now I don't know what that switch does but I took it from the donor.
My first hurdle will be trying to use my puddle lamp housing on the heated mirror since it doesn't have them and I don't want to lose them.