Tailgate latch not locking, license plate lights out

The fact that you found issues in the rear SAM would be the likely cause but the door not pulling in far enough sounds like a possible adjustment issue. Also the motor in the latch appears to be controlled by the rear end closing module.

So I would suggest starting with this but you also can't rule out a latch issue with the replacement part. Did you get this from a parts store or someplace with a warranty? If so, we may need to exchange it out if everything checks out.

https://www.2carpros.com/articles/how-to-check-wiring

Let me know if you have questions or what you find with this.

Thanks for the info. I saw these same diagrams on another post but thought they didn't apply to mine because it was a W166. I was under the impression that the rear SAM was responsible for "pulling in", hence trying to figure out the pin outs.
As far as adjustment, I can't see anymore adjustment possible than what I have already done. The tailgate does latch closed, but to clarify what I mean by "pulling in": There is a second motor in the latch that should kick on after the tailgate closed and latched. It should pull tight and secure the tailgate. Think of the normal door latch, it has the first catch but there there's a second catch that keeps the door tight otherwise it kind of feels loose. This is what I am experiencing with the tailgate. The reason I think adjustment is okay is because A)the first catch works and B) I don't here the second motor being activated. I suspect a module is not commanding it.
It's possible that the new part could have a fault, yes. It's the OE MB part (from worldpac). Warranty is not an issue, however stock is.
What I don't understand is the connection between the tailgate latch and the rear end closing module. The wiring diagram doesn't have any wires between them. Except maybe the canbus wires through the SAM.
Any documentation on which wire operates the 2nd motor in the latch? I see 8 wires and except for positive and negative-, I don't see what they do. I need an internal block diagram similar to the way it shows the connections on the closing module.
Thanks again for the info so far.

You are correct. I am sorry, I looked at that too quick and saw the motor from the rear end module and assumed that was the locking latch. But the rear door locking unit is the latch in the center of the tailgate. That is the 10 wires going to the rear SAM. So these are the wires that you need to inspect and then monitor voltage. I am looking for a connector view to tell us what each wire is but I have not found one yet. The OEM is not very friendly when it comes to them.

Yes, the 10pin harness, but only 8 wires in it between the SAM and latch. The wires look fine on both ends but I will do an end-to-end continuity check for each wire and voltage drop for integrity just to make sure the harness is okay. As for monitoring voltage, I totally agree with you, just don't know what voltages to look for and I can't believe how difficult it is to find these schematics. You posted some great ones but even with those, it still doesn't tell you what you need. Definitely not friendly. I've never seen a wiring diagram that doesn't tell you what the wires are.
I will wait and see if you can dig up anything.
Thanks again.

Here is some more info that we kind of already knew but it is a better illustration of how the rear SAM is involved.

It is just frustrating because there are no connector views at all from any wiring diagrams, OEM or aftermarket.

Even the OEM wiring diagrams are not helpful because they literally just show the same wires but no descriptions. Mercedes has always been this way. I am going to check Mitchell manual as well and see what is there.

Let me know if anything turns up in Mitchell, otherwise I am going to conclude that the rear SAM has internal faults and needs repair or replace. There is a code (91C8) for the license plate lights not working. E19-3/4 rear SAM internal fault or something like that.

Correction: 91C8: E19/3 (license plate lamp, left rear-end door): control unit: internal fault. CRP429 scan tool.
Both lamps are out though. E19/3 and /4. Bulbs are good, contacts are clean, grounds are present, B+ does not supply 12v.

Maybe we should have focused on that before because it sounds like you nailed the rear SAM as the issue of it. The rear SAM directly supplies voltage to the bulbs so if wiring and bulbs are find then the SAM is the issue. Add this to the other issues you have found, I would move forward with it.

Yes, I just noticed the center high-mount is on that same circuit and the car did have that out as well but then randomly it started working again. It seems as though there are some intermittent issues on the board. I recall testing the coil and connectivity of the two onboard relays (K101 and K102) and at one point it was all shorted everywhere and then began testing correctly again.
That last diagram looked like it had some more info. Am I able to get surrounding area for this diagram?
BTW, this has been a great resource so far with speedy replies. Thanks for all the help!

Okay. Sorry about that. Here is the complete diagram in 4 sections. Let us know if you need more info on this.

This has been great, thanks. Looking for replacement SAM options. Once it's changed I'll update if it worked.

Sounds great. Thanks for the update. Looking forward to hearing some good news on this one. Thanks

Just realized I never updated this. Customer got another rear SAM unit a few months back that was good. Everything works now. Rear SAM was the issue as we suspected.

Hi,

Thank you for the update. I'm sure it will help others.

Take care and please feel free to come back anytime in the future.

Joe