Hello,
first time poster here.
just did a engine swap on my s204 (om651.913) but only swaped a short block in, moved everything else over from the old engine. The car has been standing still for 2~ years cause of the engine and I bought it and was planing on using it after the swap. Now the car won’t crank or start with the key, I have shorted the starter to make sure it’s not that and checked every ground point and CAN-bus treminal in the car with nothing being corroded or bad looking. I have a bunch of fault codes but I can’t figure out where they come from.
thanks for any help

Looks to me that some plugs in car wire harness were not inserted properly.
When rear door malfunction should not disable the starter, on computer cars lot of things are interlocked, so get those things in order.

I have now gone over every connection and ground point in the car including inside the car, nothing is corroded, damaged and is secured properly, still it’s not working. Thank you for the reply, I’m still searching for the problem

Hi OP, any update on this matter?
I would first focus on the no-crank condition. Can you track the starter connectors up to the SAM? Does it crank when you give it 12Volts on the SAM?
It could be something nasty like the EZS

little update, the YAW rate sensor was completely corroded out, don’t know if it’s that but haven’t found anything else that was wrong. Still working on it but waiting on parts

I’m so so close to giving up totally. Engine cranks if I short it but not on the key. Changed a bunch of control modules and connections that I thought were bad. I have checked every connection on the car 6-7 times and alt the wiring but nothings wrong. Only fault codes are comm errors on can-bus «combustion engine/gearbox, no message…. Etc» if someone has any idea of what it might be please say I will try a little more then I’m giving totally up on the car

Did I miss your car year?
Anyway, W211 is highly-computerized car, so you should be able to do lot of tests and adaptations with a scanner.
Years ago I bought MaxiEcu, who is excelent scanner, with lot of adaptations, but unfortunately being European -made, doesn't cover some of US modules.
Still it should work on your vehicle, or you can get different, top-end scanner.