Hi All -
I installed a hitch and wiring harness adapter on my 2013 135is yesterday and figured I'd share my experience since I see a lot of the same questions being asked.
I picked up a
Curt C111841 (which includes a 1 7/8" euro drawbar) and a
Tekonsha Modulite ZCI harness adapter. I picked up the latter since it required no splicing (and should result in no light errors).
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Hitch install is just like it says in the directions included with the Curt. Remove the bumper fascia and the plastic undertray thing on the passenger side. The hitch slides right over the bumper bolts with everything else in place.
These are all the tools I used for the install. Well, plus a sharpie and a razor knife.
The exhaust bracket (this is the PE, mind you) was ~1/16" from the hitch bar. This is the view looking up from the ground.
So I marked and cut the bracket.
Cut out the plastic like it says (I had to cut further down in the center section)..
And it fits right in there.
Bam.
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Here's the ZCI harness kit:
It has inductive sensors which snap around the wire.
I started by pulling off the trunk liners, and under the right side carpet is a molded foam support which had an existing electronic module in it, and a slot that perfectly fit the ZCI control module.
I wired everything up using mostly signals from the right side harness, with the exception of the left turn signal. The inductive sensors are slightly bulky, so the challenge was finding locations for them where they wouldn't rub or be crushed in normal use of the car.
Here's the right side...
... and the left side.
Not my cleanest wiring work ever, but it's tested and fully functional.
There's a main chassis ground right near the right tail light, so I used that to ground the ZCI unit, and ran power to the trunk (orange wire in the top right of the following photo). The trailer harness just hangs out in the battery compartment.
And will hang out of the trunk when in use.
I'm not super happy about having it hanging out of the trunk, but I didn't feel like taking the bumper off yesterday. it was ridiculously hot. Maybe that's a project for another time.