Oh hell! I have precisely the same problem and took my car to the Merc dealer today, quietly confident that they'll find the right one easily. It started well enough, "well they look like Mercedes nuts" he claimed. However, no luck. All their nuts were too small (or more the the point, mine were too big). He said Merc ones are 17 mm, so mine must be 18.5 mm. I'm going to take it to a Chrysler dealer on Saturday (wish me luck), but does it sound like mine are not standard? I have four regular bolts in the boot. Is that normal?

Car ownership really is a trial sometimes...