I have a personal watercraft that I tow to launch with my X5. The lights on the trailer stopped working. I found that the power to the hitch outlet was fine, but the bulbs would not light on the trailer. I decided I would replace the lights (didn't really like the old ones, anyway), but the new ones didn't work, either. I struggled with a voltmeter, bought a new wiring harness, and all sorts of efforts, all to no avail.
Then, a guy doing some handyman work at my house walked up and asked what I was doing. I revealed my frustration. He then said, "Isn't this ground wire supposed to be attached to something?"
They work fine now.

Sometimes it helps to have a new set of eyes look at a problem.