Mothballs always work for me - course the car has to be aired out when it comes out of storage. I also put some pellets under the car just for good measure.
When our house was being built, mice got in the HVAC ducts. I watched one go between the slats of a register - wouldn't have believed the space was wide enough. Wife didn't want me to use pellets for fear mice would die in the duct and stink. So I put traps under the registers - caught a few but didn't solve the problem. I finally told her either she let me use pellets or I put a snake down the ducts - she opted for pellets.
