I had the same problem earlier this year and the dealer couldn't figure it out. I read through the various threads on this web site and decided that I would gamble and order the relay module and I'm glad I did. I haven't had a problem since replacing it. Luckily, I have a buddy that works in a garage and he got me the part at cost ($125). He actually ordered me the part from Mercedes rather than Chrysler because Chrysler wanted $200.

I don't know if the part numbers have already been posted in this thread, but here is the part number from Mercedes:

A 170 545 03 05

Please note that the part number above is on the box I received upon replacement. You'll want to be sure to verify that it's the correct number when purchasing your own. Hope this helps!