how do you stop mice from making nests in your cars engine?

Hygiene, buy a proper car

I agree with the mothball technique, is great for squirrels and cats as well

Do you mean (in) the engine, like in the air intake/exhaust, or just under the hood? I guess get an exterminator.

A stocking full of moth balls is best if the car is not garaged but will stink up a garage. If it is garaged, use the slightly less effective Balsam Fir products. I get mine from Walmart.com. It still stinks up the garage, but at least it is a "piney" smell.

Seal up the openings so they can't get in.

Buy moth balls and fill an old sock with them. Tuck it into the engine compartment in a place away from the drive belts.

Drive it more often.

Get a cat.

charge them rent

Spray some motor oil mixed with peppermint around