I leave the hood open on all my vehicles - when parked at home. Been doing it for a year and no mice in the engine compartments. It use to be that the engine compartments were "mouse heaven". Never had them in the passenger compartment - knock on wood.
I got three barn cats this last winter. They are doing a WONDERFUL job on all the mice. I'm hoping that they will soon turn on the pocket gophers also.
With the barn cats and the dog - I won't use poison.
3R - what kind of mice do you have? They can eat holes in a metal heater core?? That sounds more like rats.