Here is an answer to one of my earlier posts, do a search on dc sockets and there is more in the replies that may help:

you should get the aux input installed and then use a tuneflex 2.0 ($10 off ebay refurb) and call it a day. Its very convienient and puts the Ipod at right height and location so you can control it (right below the volume button). Those FM transmitter ones sound like crap.
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1999 M Coupe - Heavily modified
2001 740i Sport - Heavily modified
2004 X5 4.8is - X-pipe! V1 hardwired, Invision DVD headrests, N62 Intermediate Levers Fixed!