A friend had them come through his cabin air filter. Put in a new cabin air filter with a piece of 1/4 inch hail screen cut to fit on the bottom side, no more mice inside vehicle....casI had the mice in my cabin filter too. The get in through the vents along the front scuttle (yes they fit through 1" or smaller holes!)... Clean or replace the filter. Then add a couple of cotton pads liberally dosed with peppermint oil. I also put a few more around the wiring loom where I could. I also spray peppermint oil solution under the hood, I found they'd been in the air filter box too!
You need to do this regularly as the peppermint burns off. But I've never seen trace of mice since I started doing this.