Wow, that is pretty incredible that he is able to do that! I would definitely do whatever you can to stop him, even if the solution is a hassle like putting a chair against your car every time. The cooking oil idea isn't bad, but even with access to the plate at all I'm just afraid he could damage his teeth on the metal. Sometimes you just have to go through a hassle while a dog is in a phase and likely soon they'll outgrow it and lose interest. My dog had a phase where she would rip up her pee-pad all of the time while I was at work. Since I wasn't home I couldn't stop her, and every night I'd have to come home and clean up little bits of cotton all over the apartment. It was a hassle, but eventually she outgrew it. Good luck!
