50GMC253,
We go through the same thing up here - Hotrod's idea is a good one. I find dead mice floating in the dogs' water all the time.
Unfortunately, you will never be able to seal them all out of your truck. An adult hamster (much bigger than a mouse) can sqeeze through a hole the size of a quarter. I can only imagine how small a hole your average field mouse can squeeze into.
When I was a kid, they would somehow get into sealed 55 gallon garbage cans full of grain for our horses. You could literally scoop up the mice by the grain scoop-full. You might just have to admit the problem is too big and mark it up as one of the few drawbacks to rural life.
