I have just finished install of the retrofit in a 135i with no original wires going into the mirror. Here's what's different from the E90 writeup and PDF document (http://www.keepandshare.com/doc/4229...m-1-3-meg?da=y
The retrofit kit (same P/N as above) contains: a cable with five wires (those that correspond to A2-A5 in the PDF plus a ground wire with a plug on one end only), empty X14292, empty X14288, another empty connector, and a plastic piece with wire knife (not sure what the proper name of this part is). You need the new X14292 and the wire knife, the other two connectors can be discarded (keep them until you're finished, in case you destroy an existing connector).
You don't have to cut A6 and A7 since they don't exist, but you have to find a place to connect that brown ground wire. Use the wire knife to piggyback on an existing ground wire in the roof switch unit. I used the thicker one that goes into the rightmost connector on the switch unit.
1. Remove the old mirror and the switch unit.
2. Feed the wires of the new cable through the hole.
3. Plug the wires into X14292.
4. Plug all but the brown wire into X14288 (unplug it from the switch unit and disassemble it first).
5. Take the wire knife connector, feed an existing ground wire and the new ground wire through it, close the connector.
7. Install the new mirror, connect X14292 to it.
8. Replace the switch unit and plastic mirror trim.