Rodents in my engine compartment, what to do to discourage them?

I live in a rural, mountain area and we have been battling rats lately, well, for months now. Have hi-tech electrical traps, birth control bait, etc. but… the damn rats have now gotten in the engine compartment of my car in the garage. Today I had an oil change and my mechanic called me out to show me the rat poop and acorn husks in my engine compartment. A few months ago I had a fuel line rupture while driving on the highway.

What can I do to keep rats out of my damn engine?