Lots and Lots of Moth Balls
My other project has been siting for years covered while waiting for a suitable soul (engine).
First thing you want to do is get rid of anything that might have a nice smell or taste. Think like a mouse, and understand what they will do to get at food and where they like to live.
Generally small cramped places, darkness is key, so these ares are what you have to look for and protect.
Remove what you can to prevent the little buggers from using seats etc as nesting material.
I pulled the seats and have them inside the house, but in the car itself I have low 1/4" high 6" x 6" cardboard boxes lined with plastic and filled with moth balls. These are placed all over the car, with attention placed where the wiring is.
For these areas I made little netting bags again filled with moth balls and hung them where I figured would be appealing to places to nest and eat for the mice.
Filled heating vents with balls aswell as they fit the above mouse requirments.
Roughly every 6 months I replace the balls. To this date, no mice or anything else for that matter bothered to invade.
Now words of caution, the interior in my senario is vinal and is sealed. I'm not sure I would use this method on a leather interior as it may absorb the chemicals used in the moth balls, but I'm sure after a certain amount of breathable time this odor would leave aswell.
I'm sure others have different methods but this seems to work for me.
Bill