Most car a/c units have filters- either take it to a mechanic shop and have them look, or look up a guide on how to do it yourself. It also sounds like you have somewhere that isn't sealed properly, because the smell shouldn't have been able to get inside your vehicle otherwise. Check the sealing on you windows, windshields, the trunk, and the front of the car. Look for missing bits of the rubber sealing, or dents in where the two are supposed to meet. After that, I'd suggest getting your upholsteries washed. If you can't afford to pay someone to do it, but have the equipment for it, look up how to do it. White vinegar is a good disinfectant, as are some other options. If you don't want to skip straight to a full blown cleaning, you can also try getting lemon essential oil, vinegar, and a spray bottle. Usual ratio I use is 1:1 for the vinegar and water, but depending how bad the smell is, you can try doing ratios where the vinegar makes up more of the spray. Five drops of oil to a gallon is what's used for laundry, so that'd be your best starting point. This is better than just a deodorizer, because it kills the bacteria, but shouldn't damage the fabric. Pat it dry with some cloths, or vacuum it dry, which will help lift off more of whatever is causing the odor.