They sure haven't worked in my house, I can't imagine they'd work in a car.
Bell & Howell plug-in ultrasonic pest repeller: Found mouse pellets directly below it.
Peppermint/mint oil repellent packs: Saw mouse actually move toward it.
Attractant gel in both humane and snap traps: Ignored.
Green poison baits in bait stations placed by exterminator: Turned the crap green, didn't appear to impact the mouse otherwise.

You know what got rid of it?
Whacking it with a plastic dustpan when it paused between one inaccessible hiding place and another.
Felt a little guilty about how long it took to actually die after I hit it (it made it under the fridge, but I was able to drag it out with a ruler), but y'know, I tried pretty much everything short of inviting one of the neighborhood cats inside.