This tutorial worked (mostly) like a charm for me when replacing the original battery (76K miles on it) in my 06 330i. The battery I wound up buying was an Advance Auto Parts 94RH7; their "gold" battery. The battery was $167.00 + tax. It's about 1.5" shorter than the original BMW battery, but featured more CCA. I too wound up with a spare wire pair that apparently doesn't go to anything I can see. One wire is red, and one is blue. I'll have the wire checked out when I have the battery "registered" in a few days at the dealership. I couldn't get the soft plastic hold down in quite the right spot, so I may have them partially reinstall the battery, even though it works fine.
2014 BMW i3 w/REX /Solar orange/ ACC and parking assist suites