This is a hard one. Once mice start using an area it is extremely difficult to get rid of them. While it may be challenging to park the vehicles somewhere else, this should be done at least temporarily to keep from attracting the mice back. Do not use poison baits or traps that are baited with peanut butter or other food stuffs as that will only attract more mice to the area. Using moth balls is a good idea, but does not sound like it is working in your case. Sticky traps (glue traps) are a good way to catch mice without attracting more to the site. You will also need to clean the cars. If you have old food wrappers or anything like that in the vehicles, that will attract mice. But also, mice are attracted to the smell of other mice...so if you find any mice feces or nesting material...all of that has to be cleaned away. Be sure to wear plastic gloves and a respirator while cleaning in areas where mice have been. If you have tried all of these solutions without success, the next step would be to call an exterminator. Good luck!
