Harness interface options
In investigating getting a wiring harness for my vehicle, assuming I'm interpreting things properly, I've found some interesting gotchas.
My vehicle, and more and more of them, are now using a high-speed serial bus to interconnect and control various functions and items in the vehicle. IOW, the computer is becoming more and more integrated into the thing, making conventional thinking and slapping something in harder and harder to do. This allows the manufacturer to not need as many wires running to things, as the serial bus (databus) can tell a smart device what it needs to do and when. It makes it harder to understand and add to, though.
As this relates to the wiring harness for integrating a trailer, it brought up two possible ways to do it. If I got this right, for the lighting functions: turn signals, brakes, running lights, backup lights, etc. at the rear, there are NO power lines to control them from the front turn signal switch...they use the CANBUS, a high speed serial databus to talk to a module at the rear which then generates the power to actually turn the things on and off. It also has logic in it to detect whether the thing is drawing current, and if not, uses that to then send a message to the computer saying that the thing did not turn on when requested which should initiate a bulb out warning.
Since the logic is only designed to handle the 'normal' load of the designated lamps, just tapping in and powering additional things to a trailer would likely overload the drivers so a different method must be employed. Those interface kits fall into two categories: smart and dumb. The factory one is smart in that it also gets patched into the CANBUS and then can tell the computer that there actually is a trailer installed, and that smart interface provides its own drivers for these additional lights. BMW doesn't sell a factory interface kit for my car in the USA, and the one they do sell for the rest of the world is not readily available here. The place I found in the UK makes a compatible version, and that's what I'm toying with buying. The ones that are available in the US are still sort of smart - they don't load the factory lights, but you need to find and clip into all of the desired functions, then when the car's module turns on a light, that high voltage then triggers the module to output 12v to activate the trailer lights - IOW, the trailer is not drawing current from the car's lights in parallel, but through the module direct to the battery via a fused link. Since it doesn't talk to the car's computer, it has no way to know you've actually got a trailer connected, and thus does not turn on the trailer stability program, enable the backup camera zoom function, or turn off the rear parking sensors. I find that last bit the most annoying. And, actually, since the smart version does not need to connect to as many places (in theory, it could live with just the CANBUS, power, and ground), it's faster and easier to install. But, as emergency backup, this one anyway, hooks to the center brake light and the right turn signal driver line. The factory guy told me that they do that in case the CANBUS dies, it can still light the brakes, and it uses the right turn signal and the center brake light line to determine that you've turned on the emergency flashers. All kind of neat and scary at the same time!
Anyway, the trailer hitch company guy told me that they're getting closer to finishing the negotiation with BMW to be able to provide a true factory, plug and play harness, but it has been taking much longer than they'd hoped negotiating all of the hoops. They're confident that they'll eventually be able to provide it, but in the interim, I'm toying with getting the Right Connections harness. This might require the use of a dongle to get to a US trailer connector or to cut off the factory one and splice in on, but that stuff is readily available.
The UK has historically used either one or two 7-pin round trailer connectors. A simple trailer uses one, a caravan (travel trailer) typically would require two - the second one providing caravan battery charging and frig power. The rest of Europe has been using a single 13-pin plug to provide all of those functions in one nice, waterproof, twist/lock connector and you can order the Right Connections harness with either one. I'm leaning towards the 13-pin connector as I like the idea of twist/lock and waterproof in something that gets exposed to that nasty environment under/behind the car. It's not that hard to build or buy a converter dongle, if you don't care to modify your existing trailer or need to borrow one.
I'll keep you updated, should I go any further, with results.
If I'm interpretting this all wrong, please set me straight!