Mouse prevention
I use mouse traps (4), one @ each tire, two electronic devices, in the car, one in the front between the seats and the other on the rear seat. Bounce sheets scattered everywhere, in the trunk, vents, on top of sunvisors (keep these down), in the engine bay. Anything that emits a strong smell (mothballs, peppermint, alcohol...), seems to detur them. My neighbours cats...
But the only surefire way, is Fastback340's, #1 method, but put something in the bottom of the tray (wood) to put the stands on, so they don't puncture the trays.
Poison is pointless, I used green poison bars once, found a pile of green rice on top of my breather.
If it does kill them, they will likely crawl back into their nest to die, that don't smell pretty either.
Good Luck!
Giles