yep Alex is correct, well actually i used the +ve from the oem loom, (think its the pink and blue striped wire) and the -ve is bolted through one of the bolts behind the stereo facia. (just under the nav screen on the left hand side if you want the exact one )
http://www.dvbcaraudio.co.uk/ipod-phono ... -4087.html
I got mine from ebay for about 10 quid but I cant find one now... here is one from a store.
Edit: Alex has pointed out the correct one