Well, the shop still has the car and can't figure out why the PBC fries. The latest news is they think the car is missing the Electronic Accelerator Control Unit and this is why the Climate Control system's PBC goes out because of some altered wiring. They came to this conclusion after obtaining a socket box tester from the dealer and hooking it up to the PBC harnesses.
I checked the schematic of the Climate Control system and do not see the EACU in the system. I really don't see how they could be related. After looking at some electronic accelerator system diagrams, and if the EACU was missing, it looks like there would be quite a few other systems (cruise control, backup lights, and more) that would not function.
Does any of this make sense? Is it time to pull the plug on these guys and take it to the MB dealer?