Left trailer signal not working
 

When I bought my 2009 X3 I had the dealership install a factory trailer hitch and wiring harness. I just sold a 5x10 utility trailer and there were two or three times were the left signal would flash triple time on the X3 but no flashing on the left trailer signal. The first couple of times I played with the wiring adapter and eventually the left trailer signal started to work and it flash at a regular rate. The third time the problem fixed itself after I started driving.

Now I have a 2005 6x10 cargo trailer with many more lights (none look like LED) and the left turn signal on the trailer does not work. All the lights on the X3 work fine. When I turn the left signal on the X3 signal light flashes a triple the speed but the left trailer signal does not flash. The right signal and all the driving lights work fine on the trailer and the X3. When I turn the hazard lights on everything works fine except for the left signal on the trailer.

I have not tried to swap the left and right signal bulbs but the previous owner said it was working with his vehicle. Any ideas?

Sounds like a continuity problem in either your trailer wiring module or the wiring between that module and the trailer.