How do I keep a Squirrel out of my car engine?

How do I keep a squirrel out of my engine? That little bugger has now made a nest in my engine 10 times in the last month. My car is used on a daily basis so it's happening every night. Only once did it do damage, but it's a pain to keep cleaning out. I can't leave my hood open because I live in a place with lots of snow in the winter. I've parked in other locations, but it still finds me! I've even had the car washed in hopes of removing any sent it may have left behind, but it didn't work. I have a humane trap set out next to where I park. I've used a sonic rodent repeller, fox urine and pepper on the ground around my car, the tires, the bumper, I've even placed it in the engine block because I was desparate! I've used moth under the hood, but the crafty little thing used the moth in its most recent nest. I know that Rat Poison and anti-freeze are both options, but I was hoping there was a more humane way before resorting to killing it. Any ideas?