The general idea that you can keep rodents from doing damage by using something that keeps them away from just one area is doomed to fail more often then not. So you keep the rodent from eating the wiring or out of your heater…big deal. There are lots of other things in a car that can eventually occupy their attention. Trapping, poisoning and generally removing rodents as they appear, is the best long term successful solution IMO. DECON works well for me but must be kept in an enclosed area with the car where only the rodents can get in, leave for water and die. We must have different rodents where I live. They use things like dryer sheets to clean up after them selves after they eat and defecate on every thing else.
So if you want to provide them with alternate nutrients they need to propagate even more and bother everyone else, at least put it in a trap.