Amazing how mice manage to get into the smallest of openings (possibly through one of the water drains or the like.)
Have you checked to make sure that all your tools are still there? The mouse could be stripping your car---piece by piece. (Check "eeek"-Bay and see if your lugwrench is up for auction
For the "humane" crowd (a misnomer for those--some on this forum
--who treat animals with greater dignity and respect than they do their fellow humans), I have a solution. A few years back, I stumbled upon what is, literally, a better mousetrap!
It's a rectangular green plastic box (approx. 3" x 6") that looks like a little house. It has a spring-triggered door through which Mickey or Minnie enters to snatch the bait (I use a cracker with either some peanut butter or some American cheese.) The mouse's weight on the floor trips the spring and the door snaps shut. In the morning, you find a healthy, well-fed mousey waiting for you to free him/her.
I learned the hard way that you've got to take the little rodents quite a distance away to ensure that they don't return for seconds (they seem to know they've found a sucker who feeds them hors d'oeuvres
All things considered, you're pretty lucky: The squeek in your Mercedes can be eliminated a lot less expensively than the one in my 420SEL's transmission