Since the mice have already been in there, your pretty much SOL. If the are limited to the HVAC assembly, you can try a few drops of hand soap in a bucket of hydrogen peroxide with a little baking soda added. I think the ratio is 32 oz of H2O2, 1 table spoon of baking soda, and 4-5 drops of hand soap. (it is supposed to work on dog urine) If they got into the carpet, padding, seats, etc. there is no simple method short of completely removing the interior and replacing. The uric acid is what gives off the smell, and it permeates the pores in the paint as well as plastic, etc. You can rent an ozone machine and try that, some people have had luck, others have not. If they got into the headliner, remove the headliner and discard.
I also recommend the scented soap. I usually grab the stuff out of the bathroom and make Margaret buy more from the house. I use the soap for storing anything as the scent keeps mice and spiders out of the cars, boat, etc. A few well placed pieces in a disposable dish works wonders.
