CLEAN the car:
1. Remove seats, soak in cleaner of choice (simple green works well) ,pressure wash, squeeze dry, place in front of a fan in the sun for a couple of days.
2. Remove carpet, see above. Alternatively, clean the carpet in the car. Hot water, carpet cleaner (simple green again) a scrub brush and a shop vac.
3. clean out the HVAC. This can be tricky. Removing it is a SOB of a project, so get some lysol and spray sparingly into the intakes with it running, switching back and forth heat/ac.
3. Clean the hard plastics. Should be self explanatory, but don't be afraid to use water. leaving undried cleaner can be almost as bad as the original filth.
4. Headliner. This is difficult to do. LIGHTLY clean spots with something non aggressive. No rubbing, just blot. Is a new headliner in the budget?