I believe anything you put in the engine bay would be smelled from the inside of the car, right? hmm...

Could try one of those ultrasonic devices.

Crap, just got some more crap in the engine bay this morning. Yesterday morning was fine. It's been 2 days without anything and now the mouse/mice have returned. Maybe I will try putting a couple of Fresh Cab pouches in the engine and remove them in the morning for a couple of weeks and see what happens...

Update:

I have not noticed any rodent activity after I applied 3 things:

1) Rodent Defense: Spray bottle solution that contains fox urine. I sprayed that around the driveway and it smelled like sh*t
2) Fresh cab in the engine every night. I take it out in the morning.
3) Some moth balls in the engine bay every night.

It's been more than a week without any rodent activities. And luckily, I don't smell any of the stuff in the cabin.

Mice are a nuisance, but that's mainly it. I was just talking to a friend in Hungary, they have a huge problem with beech martens. These cute animals get into the engine compartment at night and chew through hoses and cables. They say that a beech marten can bite through a starter motor cable with just one bite. They think that the fish oil, often contained in the cables, hoses and insulation of Japanese/Korean cars may contribute to this, but cars of all brands are attacked. What is even worse, this damage is excluded from their insurance coverage.

While mice may only be a nuisance, their droppings in quantity are nasty and can become a health issue. I know - I've been in a constant battle with them for years in my basement. No matter how hard I try to seal all the tiniest crevices, they manage to still get in.

That's right. The droppings could be potentially disease causing and the mice could chew up wires - something that could be dangerous.

Anyway, I am glad that they have not returned.

Mouse Trap
 

Don't know if you tried this remedy yet. But just in case mice return and Tom can't help you, this may work.

Rodents have returned as the climate got colder..... arhhh....

When I put the boat up for the winter, I use bounce sheets. Maybe stuff a few of them in the engine compartment. Much better smells than moth balls.