Originally Posted by Mr. Canoehead
Do you have ABS? The brake lights work opposite for ABS/Non-ABS bikes.
Note: Most 1100 and 1150 models are equipped with ABS from the factory. Versions of ABS vary, but they all route the brake lamp switches through the ABS computer. If you remove the ABS unit, you've also got to rewire the brake switches to activate the lights. BMW uses NC switches.
EDIT: replaced ALL with Most. It took some digging, but I did find indications that I was wrong. The parts fiche shows two FD PNs, one for ABS and one without. Based on that, I've got to assume a non-ABS version was available at some point.