Also I looked at having a newer Garmin (3790T),that I bought at Costco, installed by my dealer in my wife's 2010 335xi. This was because the aftermarket GPS available from BMW used the same unit. The cost to supply and install the GPS kit was well over $500. When I asked the Parts guy if I could bring my unit in he told me they had just spent two days trying to get one to work for someone else's Beemer without any success.
To find out why, I called Garmin in Kansas City who told me that their unit that is sold by BMW is a propriety one. No surprise but a bit of a pity.