I recently purchased this ML350 and have started to dive into its problems. The biggest category is electrical/electronic problems, and they are widespread enough that I suspect there's a faulty control module. I've checked half a dozen components including fuses and relays and bulbs that are all good, so before I go through checking every single one or starting to pull the control modules to narrow down the possible points of failure I thought I would enumerate the problems here and see if someone more experienced can give me an "aha! those are all controlled by the XYZ module". Without further ado...

Every exterior light is dead except the tail lights. That includes the low beams, high beams, turn signals, brake lights, parking lights, etc. The dash display enumerates approximately 14 such failures.
The cruise control works most of the time, but occasionally stops working mid-drive. When it's dead, the dash display shows the failure.
The climate control mostly works, but sometimes the driver side will only vent exterior air, no heat or cooling and no fan control. There's no dash indication for this failure.
The rear hatch power open/close doesn't work, but the "door is open" LED on the open/close switch on the driver's door does work. No dash indication of this problem.

 

UPDATE!

The right head lights were due to one bad bulb, replacing it got them both working. The left head lights are still dead with good bulbs, possibly due to broken wires, so I'm trying to figure out how to follow those since they are inside the lamp housing.

The tail lights all work individually. I pulled the housing unit so I could probe it with +12V and all the lights come on when powered. I can't find any fuse dedicated to those lights, so at this point I suspect the problem is the rear SAM module. That diagnosis would also explain why the rear door power lock and power open/close aren't working.

I've taken apart my rear SAM and it doesn't look corroded, but I have seen some water in other low rear areas of the interior and there's a hint of water residue on the back of the board. I'm wary of dropping $180 on a replacement SAM without being sure it's bad, though.

 

It's probably the rear SAM. Any codes to verify the suspicion?

You might get lucky giving it a very gentle scrub with some electrical contact cleaner. Don't forget to check/clean the harness plugs. It will cost you a few bucks and some time, but if it works, happy days... Otherwise you're going to need another rear SAM. They are programmable, so good used should work.

 

I don't have a reader to check the MB-specific codes, although if I go much farther with projects on this car I might invest in one.

I found a used SAM for a good price, so I ordered it and should have it in a week or two.

Until then, I'll be wrestling with more mechanical less electronic failures, like my sunroof tracks being jammed.

 

Youll be chasing your tail and spending by guessing on parts and your time without a decent scan tool. Launch 431 or similar with bidirectional coding for $150 will be the best thing you ever did. it will help you find solutions to multiple problems in one go.