I use cotton balls soaked in pure peppermint oil. Mice HATE peppermint oil and will avoid the smell at all costs. (It has to be pure peppermint oil.) I put a few balls in the engine compartment of each tractor, and in the cab of the Kioti and under the seat of the Kubota.
I also use a battery powered rodent repeller under the hood of each tractor and in the cab of my Kioti.
The combination of the peppermint oil and the electronic repellers has worked so far. I started using this method when I found a mouse had built a nest on top of the fuel tank (near the filler neck) of my Kioti.