Mildew smell with A/C

I have a 2003 Pontiac Montana with 55,000 miles on it. I've owned it since new. The problem I've had since new is that when the a/c is turned on there is a horrible mildew smell that comes out. I've had it at the dealer several times and they say there is no real fix for it. They spray some deodorizer and disinfectant in the ducts and is good for a short time and then the smell returns. GM says that they have installed a blower in the system that runs for approx. 30 sec. after the car is turned off to dry out any moisture that is in the ducts.
Is there any solution for this problem?

I feel your pain. I too have a 2003 Montana that issues waves of musty organic odors from the ventilation system. When I run the A/C, the odor isn't so prominant but if I turn it off or after the vehicle sits for a while without running, the odor is overwhelming. A GM dealer has deodorized it for me once but the effect was only temporary.
I assume the odor is caused by accumulated moisture in the condenser. Any other vehicle I've owner with A/C has a small rubber drain tube protuding through the firewall in the engine compartment so condensed moisture can drain out of the A/C. When this works right, water drips onto the ground under the vehicle. If this plugs, water often leaks into the passenger footwell. I see no moisture under my van of in the footwell... just a musty odor.
Does anyone know if the 2003 Montana has this drain hose in the engine compartment and if so, how do I get at it to check for a blockage?