This is a potential solution to issues with your car not starting. Sometimes the ground cable will get corroded or even damaged.
This is a very easy DIY
Part number: 12427840580
Part diagram:
http://www.realoem.com/bmw/enUS/show...06#12427840580
Tools needed:
Jack
Jack stands
socket wrench
8mm socket
16mm socket
13mm socket
13mm wrench
Get the car up on jack stands and begin taking off the thick metal plate surrounding/attached to the front central jacking point. There are x7 16mm bolts (don't forget the one in the middle of the central jacking point!) and x3 8mm screws that connect to the transmission shield.
Once the metal plate is off, you can access the ground cable. It is located by the driver side header-to-xpipe connection. One side of the cable is attached by a 13mm bolt, and the other side of the cable is attached to a stud with a 13mm nut. The 13mm nut can be taken off with a standard 1/4in drive socket wrench with room to spare (so a 3/8in drive would probably also fit), but for the 13mm bolt, there is not enough room so you will have to use a 13mm wrench.
Put your new cable in and reverse all steps. Make sure you note that the cable has a different style "plate" on each end. The totally flat side goes with the bolt, and the "folded" side goes on the stud with the nut.
.