Why does your car smell like rotten eggs when the ac is on?

The water drain line is probably clogged up and water is standing inside the evaporator case and fermenting. Find the drain hose and suck it out with a wet vacumn. Try installing clean water back into the hose several times to clean it out once it begins draining ok. (like with a enema bag and hose)Also maybe use a spoonful of bleac to kill the growth.