Take a small metal tin, drill some holes in the side. Put moth balls in there and just leave it under the car. I just leave mine out there all winter.

Summertime, I plant Catnip as close to the car as I can. It won't keep the rodents away but what it attracts will.

Funny that I see this thread now. I had an issue where my automatic transmission was all screwed up. My car would make this really hard shift every time I shifted my car from P to R or P to D or R to D. Basically felt like a really, really bad manual shift where you're grinding the clutch. I got worried so I took it in and apparently some rodents chewed through one of the transmission cables in my car

Hope you are not serious about the catnip. if you think having rats stuck in engine bay sucks wait till you have a cat stuck in the engine bay.

I had this issue with them crawling into my engine bay, didn't chew anything however my wife wasn't so lucky. I went to change her headlight bulb and when I opened the hood I noticed a nest and some wires chewed partially through. The weird thing is her car has never been parked outside so it got in our garage. I got rid of it's nest, fixed the wires and set up a bunch of traps but the fucker never came back.

I'm going to try the Dryer Sheets under my hood I've heard that from a few people, see if it makes a difference.