Husband writing this review, not that it matters. My wife just wants "to push the button and the thing works" with tech stuff, so you're stuck with me. I looked far and wide to find a wiring system that would work with a non-trailer-ready BMW (U-Haul said there was nothing out there - Ha!) I found more info than what was on Amazon on etrailer, and they had a decent video that really helped. And on their site, you put in your specific car and it tells you which harness/hitch/killer cassette stereo, whatever will work on your car. The Tekonsha 119251 kit was for ours. Went to the Tekonsha site just to compare prices, did the same exercise, and it came back with nope, there isn't anything that will work on your car. Well that's a wee bit troubling, right? I sent Tekonsha an email, got a response 2 hours later saying, yup, the 119251 is for you! Woohoo! So thanks for the GREAT info, etrailer and Tekonsha, but I bought the kit from Amazon where it was WAY cheaper, used the etrailer video to get me started, and went to the garage to rip out the innards of the back end of my wife's car. What you need to know first, is that this does work! Take your time, you'll get it. I think the hardest part is determining what setup your car is. And it's not exactly clear in the newer models. Is it a 2-wire, 3-wire, BT (brake/tail light multiplex), or a BTT (brake/tail/turn multiplex) system? On the BMW it has 4 wires going to the tail light assembly, so not a lot of clues there! I decided to start with what worked on the 2017 model that was in the etrailer video, meaning I was only going to hook up 3 of the 4 sensors, leaving out the Brake sensor which would imply (according to their instructions) that this was a 2-wire system (with 4 wires going to each light...so it makes total sense.) ???? The video said the turn signals were on a white with purple striped wire on the 2017, of course there was none of that in the 2021 model. But there WAS a purple wire with white stripe on both the driver and passenger sides. Let's give that bad boy a shot! There was also a green and blue wire in the bundle. The others were solid colors. After temporarily hooking up the sensors, I ran through the completely simple programming, attached the trailer and voila! Yeah, that didn't work. Turns out on the right (passenger) side, the blue and green was turn signal. I found that out by just trying it. Once I found out that only the multi-colored wires mattered, it was simple. Try it. If it doesn't work, switch the sensors. Try it again. It took me under 2 hours to finish once I got the panels off, and it works GREAT! Lessons learned: -Doesn't come with a trailer light tester or voltage sensor, both of which would be useful. You don't need them if you have patience and a trailer right next to your car in the garage. -The sensors are kind of big when you're trying to get them in the middle of tightly packed wire bundles, so plan to cut back on the tape or whatever is holding the wires together a little more than you think you need. -I went full high-tech during the install, using Scotch tape to hold the sensor and wire together until I found the combination I needed. Once done, took the tape off, and worked a while getting that foam tape stuff they send with the kit around the wire and sensor while not wrapping the other wires from the tight little bundle in there with them. It sounds simple, but if you heard the language coming out of my wife's car at that point, you may have a different take on it. -I used a zip tie (included in the kit) at a point about ¼"-⅜" below each sensor to hold the sensor and the entire wire bundle together. This also helps keep the sensor from sliding up and down the wire going over bumps. I don't know that it matters, but I was on a roll and was feeling it after hitting the right combination of sensors and wires, so why not? -Important note: I witnessed while having the sensors not set up right, that sometimes the trailer lights would remain on with the car completely unpowered. I also read online that this bad boy sucks battery juice all the time. So you need to either install an inline switch or just pull the fuse when not in use. I zip tied a ziplock bag (snack sized, fuses are small) to the yellow fuse wire, and have the fuse in there. Don't forget this if you're going boating! A dead battery in your car/truck will not make the Admiral happy at the boat ramp at the end of the day. I completed this conversion project yesterday, and other than being a little sore from playing contortionist (hey man, I'm almost 60), I'm pretty pleased with the kit and this whole process. The videos on etrailer are invaluable, so watch them first. Then go for it. You won't hurt your car, and if you've set it up wrong, just try again. The system is pretty simple once you understand it. I would highly recommend this to anyone who wants to hook up their own trailer wiring harness. Good luck and happy trailering.