Just as a totaly off the wall idea. find a motion sensor activated sprinkler system. they are normaly used to keep rabits and dogs and other assorted furry critters away from gardens and stuff. might work for what you need to scare away the squirrels. Only thing is the squirrels might run into the truck to get out of the water. if they do you could always pop the hood or angle the sprinkler so it sprays up under the thing. dunno really how well it would work but they seem to work real well to keep animals away from other stuff. At the very least you have a new toy to mess with johova whitnesses and other annoying door to door type people if you want.
http://www.nixalite.com/scarecrow.aspx Just one of many links i found on google for them.