Mouseproofing a van?

I have a 2002 Chevrolet Express 2500 cargo van, which has been retrofitted with automatic side doors and a wheelchair lift, and had a rear bench seat installed.

I’ve recently discovered mice nesting inside the van. I have completely emptied everything out of the van, and vacuumed out all dirt and droppings. I set eight mousetraps (Mice Cube live traps; I don’t want to kill the little creatures, just get them out of my van), four in the front part of the van and four in the rear. So far I have caught and relocated four mice, all in traps that were in the back of the van. All the mouse nests and mouse droppings I cleaned out were also in the back part of the van. So they seem to be coming in somewhere in the back.

A neighbor suggested that they’re probably getting in through the ventilation system. My owner’s manual says, “Your vehicle’s flow-through ventilation system supplies outside air to the inside of your vehicle when it is moving. With the side windows closed, air will flow into the front inlet grilles at the base of the windshield, through the vehicle and out of the rear exhaust valve.” It doesn’t tell me where to find the rear exhaust valve.

The neighbor said to look around the wheel wells for an opening. I found mouse droppings on top of the right rear well, but I didn’t see any opening.

Where can I find a diagram of the ventilation system, preferably with instructions for cleaning and mouse-proofing it?

Is there any other likely place they could be getting in?