Back in the day when I wasn’t sure if I wanted to buy the $350 bavauto basslink, I am now certain my purchase was the right choice. Before Christmas I bought it and now I'm going to do a little DIY and Review. IF YOU WANT THAT EXTRA BASS FILL, BUY THIS!!! The bass comes no where near my old two 10'' 800watt subs in the jeep. However, this is the perfect filler and doesn’t take up too much space.
First, start out with the unit...
Open your trunk and find the BMW factory amp. Take off the left tail light cover and pull the "carpet" down and you will see the amp. Unplug the factory harness, and plug the Basslink harness into the factory amp.
After that, plug the factory amp harness into the other end of the basslink harness.
After you get all of that goodness finished with

, turn the sub around and plug in 2-3 things, the bassboost adjuster, and the "main wires"...it’s that easy
Mount it with the mounting screws if you choose,

(I didn’t mount mine in the center of the trunk, don’t worry :P)
Wire up the bassboost controller to the front,
YOU ARE DONE!!! Turn on the car and enjoy the extra bass.