Mothballs...maybe.

These days I don't drive my Toyota pickup much. I mainly have it for its occasional utility and one day I hope to mod it. A few months ago, I jumped into it to head to a meeting and the engine cranked but wouldn't start. It had been a month or two since I'd driven it previously. I decided to jump into one of my other vehicles and have a later look. When I got around to working on it, I popped the hood. As soon as I did that, a squirrel came bolting out from underneath the hood and scurried off like nobody's business. I found that said squirrel and kin had chewed through the engine harness in one spot and a couple other wiring runs. I got it all respliced back together and the truck is running fine, but I wanted to prevent Mr. Squirrel from desiring to roost again. I taped mothballs along the portion of the engine harness that was chewed through, as well as in a couple other spots (away from high-heat areas) and there has been no sign of squirrel repopulation.

Now, you won't want to place mothballs on the floor of your truck. They're not really safe to handle (particularly by kids who don't wash their hands), but maybe you could put one or two in the glovebox. You'll want to approach the use of mothballs within the passenger compartment with some restraint. I'm not sure, but I think even the use of them in an enclosed area that you are breathing in might be hazardous. Check 'em out (read the fine print) and use your own judgement.