Mice - Any advice on where to look for points of entry?

So here's the quandary - the enclosed never belly meansi can't see any of the seals where the wires, pipes and other things enter the trailer. There are a number of places where something could enter the enclosed underbelly (slide tracks, air vents, etc...), but these are necessary openings and we have no idea what the holes might be that actually allow the mice into the trailer box itself.

Short of taking down the entire underbelly, is there any way to try to tackle this?