I've been running a trapline in my 97' SR5 4R Plow truck for a few years now, I don't use it much during the summer, no food in it except the necessary bait, and its clean inside, but I consistently get mice around or near the HVAC blower, I have also had to remove the blower to remove a nest twice, I keep good tabs on it. Recently this summer I had mice move into my HVAC blower on my Mint 99' Limited at work due to the commercial garbage in the area, I got them the same day they moved in luckily and removed the blower to evict them, and then began a trapline in the 99' 4R aswell and undercoated the area around the hood hinges, I also undercoated the HVAC intake area above the blower, no more mice so far, but I thought I would see if a physical barrier would be a more permanent solution. I decided to remove the wipers and hood cowling, and proceeded to undercoat that area aswell, then I added adhesive steel roofing valley gasket to these locations so they didn't have a runway or direct entry into the HVAC system. I'm convinced the undercoat works well as a deterrent if it hasn't washed off, but the physical barrier, although only foam, should be a more permanent solution, the foam really squeezes down to be solid, you can see below the locations I placed it to be most effective, time will tell, but so far all my traps remain empty.. Keeping the HVAC set on "recirculate" did stop the blower nests, but didn't stop mice from getting in. Any other points of entry that I'm missing?.



Point of entry

 

All you can do is your best. I really don't think that it is possible to keep 'em out entirely.

I've watched deer mice smaller than your thumb jump straight up more than 4 feet to get into a vehicle and,

believe it or not, seen kangaroo rats and pack rats chew holes in unopened tin cans. Best of luck!!

 

I laughed so hard when I seen this as I had the same problem garaging my 83 in the winter. The little SOB's got in a built a nest in the fan blower wheel too and started to chew on the plastic interior parts. I installed open cell foam in all the vent passages and fan inlet. Two winters ago they crawled up the carburettor intake snorkel and built a nest in the air cleaner so a piece of 1/2" wire mesh was welded over the intake snorkel, so they may be getting in there too. I set about 4 mouse traps now, two in the cab and two under the hood and a bunch in the garage baited with peanut butter and/or bacon and don't have a mouse problem since. Check the wiring and hoses cause they like to chew on stuff.

 

I hate to be a negative Nancy, but I think you'll find that foam shredded and dragged into the HVAC sooner rather than later. I've seen mouse nests made out of steel wool and fiberglass insulation is common. They'll love the foam IMO.

I've had the same problem in every Toyota pickup I've owned and probably worst in my '04. In 2018 I trapped 44 mice in it from June to October and only started then because some got into the cabin and made a disgusting mess. Make sure the fresh air vent is fully closed when parking the truck and they at least won't reach the interior.

 

So far I have nothing to loose doing this in 15 min, I use to trap mice consistently on a weekly basis, still none in either vehicle, perhaps the undercoating is also helping as well inside the HVAC Well below the wipers, also some got onto the foam making it less desirable to chew I'm sure, just like a house if you seal up all the easy access runways/openings they stop coming in.

 

If you rarely drive it you can keep a old coffee can with some ammonia soaked rags in it. That seem to work fairly well. Also pure peppermint oil on cotton balls seems to help reduce the infiltration. Steel wool is better than foam hands down but not guaranteed. Its about being proactive like you are.

A word of CAUTION if you use the ammonia rags. DO NOT EVER use bleach inside the cab. Bleach and ammonia are a very dangerous combination.