I have my wiring notes from my build I'll post later. I have a 2011 X3 Hifi. I replaced the front tweetrs and mids with dynaudio and the underseat with Jehnerts and added a sub amp and JL Sub box that I leave in the back. I used a Arc audio 6/150 amp and a Audiotec processor. Its amazing. Anyway I have used Musicar NW for most of the gear and they provided a plug and play harness for under $100 and no wires have to be cut. They know BMW's and can make a harness in any configuration for those that prefer not to cut wires. Keep in mind with the stock amp behind the left side of the trunk area cover that there is both pairs of wires, one from the head unit to the amp ( signal) and one set from the amp to the speakers. In my case I bypassed everything but the three pairs up front and the sub I added. I never touched the dashboard or headunit only grabbed the wires from back by the amp pretty easy. Imo the processor and good speakers are key in any up grade amps can be had pretty cheap.
http://musicarnw.com/ Fwiw I drive 98% of the time alone so I designed the system for that purpose.