I nominate this thread for the title of "Best Thread Ever Without Cheesecake"

Oh, and see if his Mercedes is set up like BMW's with the battery under the rear seat. If so, put it in the battery compartment. Also, one up under the dashboard somewhere will get him smelling it sooner, and when he finds THAT one, he'll think he's found it, and the resulting stink from the one under the seat/in the battery compartment will just be assumed to be the lingering smell from the one he found, and will be allowed to fester longer, perhaps to the point of being unremoveable.

For immediate satisfaction, if there's a Spencer's in your area, pick up a can of their fart spray, tape the spray button down and toss it in the back of the car. Mebbe throw a dog turd in there too so he thinks his dogs did it.
I haven't worked out how you'll get the fart can out without subjecting yourself to the punishment, but I'm sure the supercomputer that is BYOAC will be able to solve that one pretty quickly.

Always glad to help the holidays be more enjoyable
