Hi!
So, my 2015 GL350 BlueTEC is experiencing an intermittent but severe electrical gremlin, and my Mercedes dealership service department can't figure it out.
Last summer, I was towing our 30' travel trailer down the road like normal (I've towed this camper about 20k miles with the GL without issue) and all of a sudden all of the dash lights start blinking, the infotainment system starts and restarts, HVAC system turns off, turn signals stop working, parking brake says it's on though it's not, dash dims as if it's night time though it wasn't. I pull over and turn off the engine, but the light don't turn off. Dash still says high beams are on even though they're not. I also noticed that the power lift gate doesn't open. Being fairly close to our destination, I decided to just start it back up and drive. Eventually the issue stopped and all functions returned to normal.
Over the next few months, the same issue popped up again. I mentioned it to the MB service department while I was getting the water pump replaced, but they said it wasn't throwing any codes and everything seemed fine.
The last time this happened was recently (at my house thankfully), and this time the electrical systems would not shut off no matter what I did. Light and wipers stayed on until the battery ran down.
This time, the MB service department said they thought it was the starter module, and replaced that (about $950). The next day the same issue happened again.
I took it back to MB and now they are saying it's related to the rear backup sensor, which is back ordered...
At this point, I don't have a lot of faith in the MB service department, but being that I live in a small state, they are pretty much the only one around.
I'm clearly no expert, but the issue's intermittent nature seem moisture related, and possibly linked to one of the GL's many control modules.
Any ideas or insights into what the heck is going on would be greatly appreciated! Other than this issue, this vehicle has been amazing.

Other random bits of into:
- About 95k miles
- Battery was tested after it was recharged and deemed good to go
- Tekonsha P3 brake controller is connected for the trailer's brakes - this doesn't lose function while the issue is happening
- MB says there was evidence of water around where the battery is housed (under front pas seat), but it looks like it was a long time ago and they didn't think it was related

 

I had a similar problem with my last vehicle, and was an infiniti. Ended up being moisture in the trailer plug, and was causing a short.
A buddy of mine had a similar issue with his toyota, where the plug was physically damaged from backing into something, and caused issues to the same degree, as some wires were grounded or touching.
Worth looking at that, and the wiring for the trailer plug going into the body.

 

It's a long shot, but note that there are TWO batteries on the GL and a failing AUX battery can cause all sorts of odd 'gremlins' even with fulling operating electrical system including a perfect main battery.

 

I would agree, Auxillary Battery, located under the rear hatch.