honestly In think it depends on YOUR MICE, as what works for one person many times doesn't work for others(as example's , in MY area, mice will use dryer sheets to make nests and even eat the stuff, same goes for using Irish spring soap and a few other that work for others but not for me here)
and this is why there are so many home brew and commercial ways of trying to keep them out of machines and places not wanting them, there IMO is no 100% BEST WAY to do so, you have to experiment and see what works best for you and your mice!
I personally have best luck with mothballs, I buy the ones in small bags , and place the bags all over inside of cab and in belly pan and engine bay area
I also add commercial mice BAIT to places where I know non targeted critters cannot get it, like inside cab, in belly pan and such(there are also tons of home made mice baits that you can make that are NON toxic to pets, if you have pets)
mice cannot handle simple things like, baking soda, instant mash potato's and so on, if they eat them, they will die!
so mixing things like this with other foods they WILL eat can help kill off mice and not harm other non targeted critters! like your dog or cat(pending what you mix it with of course)
IMO< one of the best suggestion I can think of, honestly is to TRY and park machine as far away from any food source for mice,
like near where grains are stored , bird feeders, or other foods that can attract them, and places they can hide and sleep,
or you will be fighting a loosing battle IMO
if you can eliminate the reasons WHY there near your machine, (things like food, cover, water)
you up the odds in keeping them out of your machine's and things!
I also recommend making a gravel or better yet concrete pan for parking machine on
over say grass or dirt
this also seems to help keep mice away
as there are also somethings you can try spraying about the machine and ground it parked on,
to help try keep mice away, they dis like certain scents, which you can buy and spray around thing or ion things or on things!
I say spray around and under machine, due to some people will also dis like the scents in the machines themself, but it can be sprayed there as well so when having to use machine, you don't have to be stuck in a over scented machine while working!
do a simple search and you can find many scents they dis like! but using scents of any kind, requires re applying as needed, as once the scent looses it strength, it opens the door again to the ice!