Get the Draw-Tite hitch (around $180 from your local NAPA store) and install it yourself using common hand tools and two large Torx-drive bits you can get at Sears or borrow from the local garage. I did spring for the BMW factory trailer wiring harness, which includes a circuit board interface and is pug and play with no splicing. Install it while you have the rear bumper off to mount the hitch. Its $199 but will save you several times that in trying to adapt someone else's harness to BMW's "unique" wiring.