We had one in the wifes car recently. I was convinced it was rats due to the mess and stuff it ate through, thank fuck it wasn't, a whole other ball game those. Surrounded by fields so get the odd one now and again, peanut butter and old spring traps always work, thankfully that's all it took in the car too, he'd got in behind the glovebox.
Fella at work told me if it was rats, or if I couldn't get them out for some reason, he could get ferrets in as he'd used them before for someone (he works with cars). Apparently they are vicious buggers, and will always flush them out, but he said it would be a last resort as if they piss in your car it's a horrendous smell that's very hard to shift.
Took the wife about a month before she would go near the car again.