On the 210, the PSE is located next to the battery under the seat and can get wet. On yours, the good news is that the PSE probably isn't your problem.
If you look on page 47 of this
http://www.mercedestechstore.com/pdfs/507%20Systems%20I/507%20HO%20CAN%20B%20(ICC)%2010-28-02.pdf
you'll see the CAN-bus junction located under the doorsill at the A-pillar (well concealed under a spaghetti pile of wires). I troubleshot my gremlin by disconnecting these one at a time, disconnecting and reconnecting the battery each time, until my problem was isolated to the rear door module.
I had to drive around a few days after each node-pull, since it took awhile for the gremlin to propagate.
No idea if this is risky, but I don't think so - do at your own risk. For me it was better than paying a dealer for questionable results (like you got).
Note that the LCP is the window button switch assembly, which I have heard is another frequent culprit.