They like to come in thru the air intake for your heater , top of the back of the engine compartment. put screen across there. They like to live in the heater core and ducting, it is not to hard to get there or the blower motor to inspect. If your car is new maybe the dealer can help. This is nothing new to Subaru. I don't know why mice like them but they really,really, really do. We had to strip the cars down to a bare shell to clean them out. They like to chew on the silicone spark plug wires. Even if you kill them they still urinated all over inside of your car and usually lived in the heater so it smells every time you turn on the heat. We removed that and presure washed them. WE even had to replace all of the felt sound deadening that is behind everything in the whole car. Bait works. I think maybe your just getting new mice. They crawl under/thru the the door rubber trim too.