Don't cover the smell, eliminate it.
First, clear the drain tube from the evaporator/heater core compartment with a straight length of coat hanger, none of the bent parts. If you get a moist musty black substance on the end of your wire, you found your problem, eliminate the mold with an enzymatic cleaner designed for the task.
If there is no mold in the heater core area, the next best thing is to remove the covers to the a.c. vents if possible and snake out as much dirt as possible. Steam can also work, but it has to be hot enough to kill bacteria, and strong enough to reach everywhere before it cools to a non-killing temp. The only problem with this method is the old, dead bacteria and mold can become food for new bacteria and mold if the conditions that started the cycle reappear. Usually this is the introduction of moisture into an area that contains an ample food supply. Organic debris like dead skin cells, pollen and plant debris are the usual suspects, so removing the food source and moisture is essential.
The problem with cover up bombs is that the longer the scents last, the worse the true bacteria/mold build up can get, so when they inevitably fade, the problem returns with a vengeance.
