Best way to keep squirrels out of my vehicle's engine compartment?

I have a consistent problem of finding sticks and squirrel nests in the engine compartment of my vehicles parked outside. I've had to replace the wires twice in the past two months. I've tried moth balls and Squirrel Stopper (worked temporarily), but they keep returning. I'm looking for a more permanent solution. I check the engine compartment daily only to find sticks scattered all over and new nests started. Last October I found 4 babies in there. I removed them and put them in the grass nearby. Momma squirrel retrieved them and ran off with them almost immediately. Please offer successful help.