It's prewired. Do it yourself. Do a quicky search and you should find a link to the DIY for it. Takes under an hour I believe.
Cost: @$230-270. Check pacificbmw.com or circlebmw.com. Their internet departments. You do need to have the dealer activated (unless for some reason it was preactivated) and the dealer usually charges for that. Try not to pay more than 1/2 hour labor charge cuz it takes like 10 minutes for them to pull it into the service bay, hook it up to the computer, and back it out again. If the Service Advisor is nice, he'll just do it for free since it's so easy to do.