I had the same issue with my BMW in 2007. The problem is a VERY complex electrical system that freaks out when additional load is put on the system and sends warning signals and causes lots of problems. I would recommend the dealer as it will be a warrantied item in case it causes problems or fry's other components.
PS - The F150 with a built in brake controller is SO much better, cleaner, nicer solution. Why can't the germans offer that for $5k or something ridiculous like other options.