I have the rear ones working at this point.
The kit I used is the LH-811C one here: http://www.amazon.com/gp/product/B00YE9 ... ge_o01_s00
Edit: These seem to have good resolution close up, much better than the ones on a '14 3 series. The ones on the 3 series however have about twice the range. In both cases, the ideal setup for the other vehicle.
It'll need to figure something out to make sure the beeping happens every time I turn on the car, but can be cancelled (until I reverse again), and perhaps something to auto-mute past certain speeds. I'll have to see how it works out in the real world once I have the front ones wired up as well.
I initially thought that I should have done them when I did the rear camera, but the camera goes in the door and these things are in the bumper, wiring ends up running differently regardless. It's a lot of wiring. Much more than a rear view camera.
Splicing the coax with twisted pair seems to have worked fine. My previous splice was shorted somehow.
I'll follow up with some pictures, but I'm happy enough with the install. The sensors are not all at the same height because they need to be at the point that's furthest out. Once you see how dented my (OEM step) bumper is you'll know why I'm doing this work.