I can help you with this as I did this retrofit to my E46 M3 in January. The cars aren't wired, and the seats do not have the elements in them. BMW aren't silly they won't waste money by putting in wiring for an option not selected as it costs them money.
Here is the DIY post for the E46
http://www.m3torque.co.uk/showthread.php?t=1111
I am thinking seriously about doing this to my mother's E90 320i and got the EBA guide this morning. It looks straight forward.