I was too cheap to buy the BMW trailer circuit card for $200 bucks and
just had the trailer hitch shop splice the trailer lights in. No problem
with single Jetski trailer.
When I got a new double ski trailer it has a lot more lights due to regulations.
I then got erratic performance. Sometime on, sometimes off. Plus I'd get
that burned out light bulb indicator on dash.

From what I've heard if you splice it in the signal voltage from car is not that strong, it is just what it is, signal voltage. They wire the lights some way
to main electrical to get power to drive lights. I think you're supposed to
get some majical module ( I think they actually put that in my system) to draw
current from battery with cable.

In the end I just bought a brand new set of trailer LED lights that have virtually no current draw versus std incandescent. Wa La..........no more problems. They work great.