No experience with it myself, but one of my friends uses dryer sheets to repel rodents. Mothballs may be more powerful but you really can't keep a car with mothballs in a garage - it stinks up the garage. Maybe mothballs to drive the critter out and dryer sheets to discourage squatters.
You really don't want to use anything that kills him where he is - a rotting carcass would not be a step up. And he won't come out no matter what you do until you have left him completely alone for a while. make it unlivable and he will leave in the middle of the night.
Seems odd a rat would take refuge there unless car is not in use?
If so move car.
Otherwise covering and gassing us best. Here a recognised rodent expert is needed.
Light a few boxes of cheap incense sticks under your car all at once.
Tie in a bunch works better and smoke him out.
Run a hose from the exhaust pipe and asphyxiate him .
EDIT: Hey , what about running a blow dryer into the vent and sweating him out ? Or a leaf blower and blow him out , lol !
I have tried rat bats also with peanut butter. I also tried rat killer pills but no success
