How to Mice in Cars
Trying to treat a car for mice in the car itself is something of a fool's errand. The reason for this is that there is generally nothing in the car for the mice to eat. As with all living things, they need to eat. So, if they cannot find food in the car, then they need to leave the car to get a meal. If you simply treat the car, then other mice are going to come back and take their place. Let's face it, they were not placed there by the hand of God. They got into the car from the ground, and they will continue to get into the car from the ground. Therefore, if you want to get rid of the mice in the car, you need to get rid of the mice that are coming from the ground. Since the mice in the car must return to the ground, you don't need to treat the car at all. What you need to do is eliminate the mouse population that is in the area where the car is parked.
If that reasoning makes sense to you, then let me tell you how to go about eliminating the mouse population on the ground. So, the first thing you want to do is discourage them from being there in the first place. If you have pets, do not feed them outside and do not leave food or water bowls outside. If you have any bird feeders, then it's a good idea to remove them. While the mice may not make their way up to the bird feeders, the birds can be quite messy and some of that bird feed may find its way to the ground, where the mice may eat it. Make sure that all your trash cans are covered, and they have no holes in them. In other words, make sure there are no food or water sources that they can get into. Also, try to eliminate any hiding or breeding areas on the property. They could be in wood piles, garden debris, under sheds, and that sort of thing.
Now that you've discouraged them from hanging out there, it's time to eliminate the population. To do this, you need to use a bait and bait stations. I will provide you with links to these products. I use a single feed bait. If they eat it just once, they die. Just place a couple of bait stations next to the front tires. One on each side of the car. When you go somewhere, just put the stations in the trunk and place them back out when you get home. If you own your home, then you can place them near the house while you are gone. You need locked stations, so kids or pets do not get into them. You need to check the bait stations every day and replenish the bait as needed, until you are no longer seeing the bait being eaten. After the bait is no longer being eaten, the population is gone. If you think they are also under something like a shed, you can bait those areas as well. Now, they may be back from the neighbor's yard, so keep an eye out for them. If you start to see activity again, then repeat the process.
Mouse bait: https://www.domyown.com/contrac-soft-bait-p-18060.html
Bait stations: https://www.domyown.com/mbs1-mouse-bait-station-p-2411.html