This is very common with air con. systems in cars. What you are smelling is condensed water trapped near the evaporator that has gone stagnant. If the evaporator drain is clear, (it's this drain emptying that causes the puddle of water under the car in really hot weather) then this smell should not be able to get really bad. If the drain is blocked, then the problem can get really bad. You can also help avoid this if you switch the air. con. off at least ten minutes before arriving at your destination. If you switch it off, then the moisture has time to evaporate, which avoids the smell.