Plenty of DIY's out there, I just searched and followed one.

Cut an old phone cord, soldered some D-sub pins that I bought from radio shack, tapped into the power source for the rearview mirror. It was a tight fit, I had to flatten the pins with pliers to get them in. A smaller version like a mini d-sub would be better, I think someone else used solid copper wire or something. Probably a good idea to wrap the exposed ends in electrical tape to make sure they don't touch each other and short out the mirror. The pins have to be put pretty close to one another. Either way, here are pictures.