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.