Since the remote locks don't work, I'm wondering if that module is pulling the CAN bus down. As our guy said in post 2 start unplugging the CAN wire on anything you can find.

When you use the metal key to lock / unlock the door, do the other doors automatically lock / unlock? ( I'd try this with the trunk and windows down in case something bad happens and locks you out. )

Given the trans computer on these cars tends to get wet with trans fluid from a bad transmission plug bushing, I'd have a look there. It might not be the actual source of the problem but it is good PM regardless.

That SDS system might not have wiring diagrams.

I've seen 2 CAN bus style on Mercedes so far. 1: Daisy chain, with an end of line resistor at the modules at each end. 2: Star, where all CAN wires connect to a common point, I'm not sure if each module uses an end of line resistor.