I have an 08 C6 that is driven daily. There is a musty smell coming out of the vents when the A/C is on. I have had it to the dealership 3 times for this problem. They have sprayed the microb killing chemical through the system. I have also had them add the blower module so the fan will continue to blow after shutting the engine off. The smell is still there. The last time it was at the dealer for this was last month and the smell is back. I also just changed the cabin filter. Is there any other options or suggestions on curing this problem. Covering it up with deoderizers doesn't work and its a powerful smell that is difficult to ignore. Thanks