Keep us posted on your progress. But if the X works fine with and without the trailer hooked up, and it's just the trailer lights that are off, I would have to question the wiring at the plug. There is a "standard" way to wire a trailer and a recepticle, but someone may have gotten either of them incorrect. I always check the pins on the receptacle for function, then make sure the same pins in the trailer plug correspond.

Keep in mind on most trailers that the right (and left too) brake light filament flashes to make the blinker. So if the filament was bad, neigher brake or blinker would work. I know it's stating the obvious, but did you check the bulb? Simple way to check- swap bulbs side to side. Also check the right side trailer light ground wire.

The other thing to do is take a circuit tester or a volt meter, and with your right blinker turned on (trailer unplugged) check for current flashing at all the pins in the receptacle. If you have current, take a jumper wire from that terminal and start touching to the trailer plug until your right trailer blinkers flash. Then see if the pins correspond.

All simple things to check, and by process of elimination you should find your problem.

__________________
'04 E53, 4.4 Sport
'97 E39 528i
'86 911 Carrera, track car
'96 BMW R1100R