I installed the Curt 111843 hitch on my 2012 135i. The 3 on the end of the model number just means it comes with a ball mount, pin and clip. Otherwise the hitch is the same as the Curt 11184. For some reason, Curt says it only fits up to 2011, but it fit my 2012 fine.
Installation was pretty easy. On M Sport models, you can get away with just removing the lower gray bumper fascia. Just remove the screws and pull down and out near the tabs to remove it. To put it back give it a good whack with your palm. Also I just left off the center lower plastic panel entirely so I didn't have to trim it.
[Edit] I went back and trimmed the under bumper panel as shown in the last pic.
I didn't trim the exhaust hanger, but I will if I notice any rubbing.
I haven't done any wiring because I just intend to use the hitch with a cargo carrier and bike rack for now. For the cargo carrier, I got a 1.25" to 2" receiver adapter with 4" rise, otherwise I think it would sit too low and scrape on inclines.
Kuat Transfer V2 Bike Rack
The Kuat bike racks for 1.25" receivers are a bit strange in that the hitch pin hole is placed in between where a 1.25" or 2" hole would normally be so that it can be used with either sized receiver. However, this means the holes don't line up and a standard 1/2" hitch pin won't fit! You have to use their included 3/8" hitch pin, but it's really long so it can slide from side to side, the retaining clip only clips on the end and not through a hole, and you can't use a standard 1/2" locking hitch pin.
Luckily this
MaxxHaul 50563 Threaded Hitch Locking Pin is thin enough to fit through the misaligned holes, and is also cheaper than the Kuat locking pin. You can just ignore the threaded part, or I put an M12 nylon lock nut on to stop the pin from sliding side to side, as shown in the photo. The other side is held by the retaining clip.
