kenvaux

2000 ML430 with 100,000 miles. Rear wiper quit working one night in a rain storm in downtown Vancouver traffic (when else). Opened the rear hatch, nothing to see, shut the hatch and it started working. Suspected a bad plug connection or a open commutator segment in the motors armature.
Next day, removed the rear panel, you have to grab it and literally rip it off with your hands as the design prevents a panel removal tool from releasing it from the clips. You will have to take a trip to the local auto parts house to get some new panel clips as some are sure to break. The interior lock handle cover is removed by removing the one screw and sliding the cover up 1/4 inch to release it from its tabs.
All the connections seemed tight and clean but noticed a relay attached to the wiper motor. I checked the number on the relay and found several in the fuse/relay block in the engine compartment. I swapped the wiper relay with the window relay and the wiper works flawlessly but the window occasionally has issues.

mihir_d

I think, more than the relays, the fuse boxes on these MLs start having issues 8-10 years into the truck's life....

I have to do the same - swap relays with other relays & confirm with the horn or window relay, that it is indeed the relay that is the problem.