What kind of bimmer you have?
Man, German vehicles can be SO hard to work with. Many are amped, and wired specifically for the stock deck. If you don't have much experience with this sort of thing, don't be afraid to get it done professionally.. BMW's(depending on year/model) are not often easy to work with for anyone, especially amateurs. This is in reference to wiring. The panels on German cars can be a joy as well, lol.
I have an Audi A6, it falls into the above category for sure. Good luck.