A mouse can push/pull itself through 1/2 inch wire mesh (sometimes known as hardware cloth). I know because I watched one do it. I had mice invade my backyard composter, which was made of some kind of thick composite material - they just chewed right through it. So I decided to wrap the composter with 1/2 inch wire mesh - did a great job, nice and snug, no gaps. After a couple of days I saw mice in the composter again. Poking around to flush them out I saw one actually squeeze itself, head first, through the 1/2" mesh. I had to switch to a 1/4" mesh. So any holes as large as or larger than the 1/2" mesh is easy for a mouse to crawl through. That's a pretty small gap to have to find as a mouse entrance into the vehicle.
You might want to look into something like rodotrak.com which is a cardboard station with cotton and UV powder. Mice will get the cotton for nesting, at the same time getting coated with UV powder which leaves a trail that can be seen with a UV flashlight. The idea being the mice will create trails one can follow to their entry hole(s). I'm sure one can find just the UV powder and flashlight online, and come up with your own tracking station. Reducing the mouse population in the vicinity with simple, spring loaded mouse traps bated with a dab of peanut butter also helps.
Good luck with getting rid of the mouse problem.