Any time I have towed a significant weight, ie 3500 lb, it has been with a trailer with surge brakes (the ones with the master cylinder on the trailer coupler).

If I was going to use an electric brake controller, I would follow the instructions on the second of the two links shown. It seems that picking up the signal from the LCM (the one going to the third brake light) would be pretty straightforward compared to building a bracket for an additional switch on the brake pedal - that just introduces another potential failure point.

The bigger question to me is where to mount the controller, given that it will be visible. The ashtray idea sounds reasonable.

Good luck with it.

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2007 X3 3.0si, 6 MT, Premium, White

Retired:
2008 535i, 6 MT, M Sport, Premium, Space Grey
2003 X5 3.0 Steptronic, Premium, Titanium Silver
2002 325xi 5 MT, Steel Grey
2004 Z4 3.0 Premium, Sport, SMG, Maldives Blue