Thanks so much for this thread! I bought my 2010 135i because of this thread (I knew I needed a hitch, and was hesitant on the 135i because BMW didn't supply one).
A local muffler shop did the installation easily for about $200. I haven't yet had the wiring installed.
One thing about the 2010: When the car warms up, the muffler lowers. There is a bracket on the right of the muffler that ends up resting on the towbar, causing a terrible "grinding" sound when accelerating. Bending the bracket slightly away from the towbar solved this.