1. Clean your car as thoroughly as possible.
2. Get out any trash and items inside your car. Clear it completely out, making sure to check under the seats.
3. Wipe the car inside with cleaning product. Vacuum the rug and upholstery inside the car.
4. Shampoo the inside of the car. Open the windows to air out the vehicle.
5. If the odor is due to mildewing water leaks, then you may not be able to get out that funky smell until you can stop the rain water from coming in.
6. Spray carpet with Lysol for mildew.
7. Armed with a spray bottle of a quality odor neutralizer, begin by opening the hood of your car in a well ventilated (preferably outdoor) location.
8. Start the engine and set the parking brake for safety. Next, place the AC and FAN on full. Important: make sure the "fresh air/recirculate" setting is in the FRESH AIR (outside air) position.
9. Go to the front fender side of the car (usually the passenger side) and look in the engine compartment and find the ac/heater AIR INTAKE, sometimes called the cowl vent. Different vehicles use different air intake vent configurations but all are just below the bottom edge of the windsheild, normally on the passengers side, and all have some sort of screen, grid or louvers covering the intake.
10. Liberally spray the deodorizer directly into the intake screen.
11. After a few sprays, get back in the car and smell to determine if the deodorizer is circulating through the AC system. Repeat if necessary. This application should treat the air conditioning EVAPORATOR, BLOWER and DUCTING..the places where smoke and odor particles will collect.
12. Now, switch the AC/Heater control to FULL HEAT, leaving the engine run, the air control in "fresh air" position and the FAN on full.
13. Go back outside the car and repeat the spray treatments into the air intake as you did in step 4.
14. By switching to the HEAT position, the deodorizer is now treating the HEATER CORE, BLOWER and DUCTING..another place that odors will hide.
15. Repeat spraying if necessary.

Tips

• HINT: If your having trouble locating the air intake screen, shut off the engine and place the key in "RUN" position. Check to make sure the AC fan is still running. Go to the front side of the car, usually the passenger side, and listen for the movement of air and the blower running. After spraying and treating both the AC and HEATER, shut off the engine, close the hood and let the car sit undisturbed for a couple of hours. This will allow the deodorizer to "work" on the odor.
• Don't use Oust in large quanities or it will pool somewhere in the vent system and leave a bad smell over night when you turn on the air/heat/vent the next day. Use Lysol (the old-fashion kind) to kill off that smell.

Warnings

• Caution: Many AC/HEATER systems use vacuum to operate, and the running engine is the provider of the vacuum. There is an inherent danger of injury when working under the hood with the engine running. Never perform this procedure with children nearby. If you do not feel comfortable doing this procedure, take your car to a mechanic or repair shop. They should charge you very little (if anything) for squeezing the spray bottle a few times!

This article was orignally posted here.