Check here to find the appropriate power and ground
http://www.bimmerfest.com/forums/sho...32#post6112632
If you don't want to buy one of the pre made wire tap harnesses (i.e. mirrortap, etc) - it is pretty simple to make your own. For the actual taps, I used a hair pin and a Dremel to strip the paint off the pin and to cut it down to the proper length. Since the hair pin is extremely thin, it slides into our mirror harness much easier than some of the pre made mirror taps, giving it a very easy and solid connection without needing to jamb in one of the thicker taps that may stretch out the back of the harness slots. Just solder the wires directly to the taps, cover them with shrink tubing and if you want to protect the circuit, you can solder in a small blade type fuse or a diode type inline fuse.
Here is what my harness looks like installed
Here is what it looks like from the side
To keep it looking OEM, I slid the wires into a cable sleeve (used for modding PC power supplies). If you look at this close up, you can see the fuse I soldered and covered with shrink wrap
To help get the mirror's lower plastic cover off, I included some pics of what the inside tabs look like
Here is what my final install looks like - I ran the harness up to the top of the headliner to my passenger visor
Have fun - definitely a good mod that makes the install look clean.