2006 R350 is missing trunk button to close and lever doesn’t work.
Can you hotwire the switch? does it work?
Anyway, So if you search this forum you will find lots of issues with the rear latch on early models. Either a SAM issue or a new latch needed. . If you have lots of issues with this car I strongly suggest you get a a shop manual. The easiest I found was alldatadiy.com Even with that it is very difficult to troubleshoot the modules without connecting to a dealer STAR system.
I have a 2011 subscription and there is not a good electrical diagram of the rear door systems. Even with out power liftgate, I presume the switch is still connected to the body sam.
As for parts, wrecker parts are still rare but you may find some. But simply replacing the buttons may not fix this. Maybe they tried to hotwire the thing?
You have the rear body control SAM, rear hatch closing control module, latch, position sensor, hydraulic pump, the safety touch sensor on the perimeter. Who knows what the problem is.
Sorry if i cant be more helpfull.






