I'm having the same problems with my '03 ML500, with a brake controller installed. I had no problems with the same trailer and my '99 ML320. After sitting, the brake lights work fine - once. After you let off the brakes, the lights won't come back on nor will the directionals work (actually, the directionals do come on, but so dimly as to be not seen). The electric trailer brakes seem to work just fine, it's just the lights which are the problem.

I've noted that the brake and directional controller box on the trailer wiring harness becomes very hot. I've checked and rechecked all connections and found no problems.

Any suggestions?