how do get rid of them think they are in my heater box

decon, peppermint oil, dryer sheets, moth balls, traps.. or a cat>>

Leave a radio on in the shop.

I've had good luck with glue traps and De-Con. I usually put them out in late summer before the rodents start coming in.

http://www.victorpest.com/store/mouse-control/glue-traps?utm_source=bing&utm_medium=cpc&utm_campaign=Bing%2B-%2BMouse%2B-%2BGlue%2BTraps&utm_term=glue%20traps%20for%20mice

They'll make a mess of a heater box in a very short time.

My car was stored in a country barn for many years and became a mouse condominium. I've vacuumed carcasses and nests out of my trunk lid and frame rails. First time the car was started in many years caused an exodus of biblical proportions. Luckily we pushed it outside. They will eat your wiring and pull cushion material out of the seats to line their nests. Poison works but they hide and die and then you deal with stink. Traps work for one or two then the rest get wary.
Pull the heater after blocking the vents, pull it out and apart to clean out the mess. You do not want to smell air passing through it with heater or defroster on. Not pleasant.

Been there, done that, you are going to have to pull the heater box and ducting. Good chance they are in your cowl area as well so make sure to vacuum the poo out of that area. For everything you can take out, I have always had good luck cleaning them with mr clean disinfectant multi purpose cleaner. Your wife probably has something similar for cleaning the kitchen or bathroom. Never had any mouse smell after cleaning.

Make sure its Rap music

Begin by taking out your heater box to start to clean it out.

Then post your property that says no mice allowed!

Doesn't work on hood rats

Or worse yet, make it country music........