I've used the same adapter as n2red to pulll UHauls in the past. Has always worked fine for me. My hitch and wiring are all factory BMW stuff.

If all the contacts in the receptacle were truly dead, meaning no blinkers, brake lights or running lights, then I have to think there's a dead ground somewhere. But I don't know if you were trying to make the trailer lights work or if you were using a circuit tester to test each contact in the X5 receptacle itself. It none of the contacts at the truck were working, try running a new ground wire from the plug to the chassis.

But if you and the UHaul guy were trying to just make the trailer lights work and couldn't, then I would have to say the trailer wasn't grounded to the truck at all. I've had UHauls where I had to physically run a ground wire from the trailer to my hitch rather than rely on the ball and coupler as the ground contact.

Regardless, my first place to check would be for a ground.

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