I have the same problem. At first I thought it was related to the recent addition of the trailer hitch because of the module that is added to control the trailer's brake lights. 2 weeks ago I removed the module. It did not fix the problem.

Took the vehicle into my highly trusted Indy and told them about it. It only happens when the headlights are on. They cleared some codes - effectively telling the car "the lights are fine". Next time the auto-lights engaged at dusk, same problem.
>Check Brake Lights< followed by >Check Rear Lights<. I'm at a total loss on this particular problem.

'02 X5 3.0i with 152,500 miles.

Anybody figure this one out yet?