Lyn might not be too far off base. Spray some cat urine around under there and see if that helps. Exactly how the urine is collected is an exercise left up to the student. I do know from experience that male cat urine can be very repulsive, but don't ask how I know.
Try this one, go to a pet shop and see if they will give you bedding from a ferret.
Even better, get the ferret. The problem with ferrets is they like to run away. Also they are not great for keeping their mess in the litter box. Otherwise they are very cool pets. They will kill any rats they meet, and their favorite thing is hunting in all the crooks and crannies they find. If you just loose him at the car, he'll just wander off. If you put his bedding under the hood, away from moving parts, I don't think rats will come anywhere near. Ferrets have very strong scent, especially the males.
I am not saying to have the ferret live under your hood, just leave his blanket there! If someone sells 'ode de ferret' get that.
Rat traps next to the vehicle. You are not going to be able to prevent them getting in unless you have a sealed building (and even then they will find a way).