I would look for the evaporator drain line or lines and make sure they or it is not kinked. Sounds like what is happening is bacteria is growing in the bottom of the drain box. Flush it out, becareful to not overflow or you will get inside of car wet. you can use some small amount of clorox water or anti bacterial soap water to flush. Just remember that clorox and soap are corrosive, so don't over flow on carpet,etc. Drain line may be the problem( plugged or restricted).