I boondock most of the year and there are times that I don't start my truck for over a week and one day I noticed that the little mouse gremlins were making a nest in the engine valley. I did a lot of internet research and came up with
Fresh Cab which I put a packet in the engine valley and replace the pouch every couple of months and I also use
Peppermint Oil that I put a couple of drops on a cotton ball and stick in the engine compartment by wedging in some of the tight spaces to hold the cotton ball in place. I use about 6 cotton balls and re-scent with the peppermint oil every month.
So far, after two years of using these methods, they have worked good . If I find some mouse or vermin evidence I will put out some mouse traps with some peanut butter for a few weeks.
I hope you find a method that works for you because those little critters can cause a lot of damage at the most inopportune times.