Musty sour smell from vents in winter (no A/C)!

Hi folks, I just bought a 2004 Prius with 35k miles from my local Toyota dealership. The interior smelled fine for the fist few hundred miles but suddenly there is that super foul sour musty smell coming from the dash vents. I know that using air conditioning condenses water and can cause mold on the evaporator coil; but being winter time, I've kept it off 99% of the time exept once or twice to defog, then turned it off right away.

I am very diligent about keeping it on the "recirculation" setting most of the time since I can't stand breathing traffic exhaust. It's been raining a lot recently and the inside windows have been fogging up a lot. A couple of times after sitting, I've noticed very heavy condensation in the interior windshield below the front defrost dash air vent. What are the odds there is a water or coolant leak in the dash? Or could this simply be caused by high humidity from rain, leaving it set on "recirc" and running almost no A/C at all??

I think I will first try cleaning the e-coil with the spray I've read about. Should I use the product the Toyota dealership sells? Or is there a generic version of this sold at auto parts stores that do the same thing? I'm hoping that I can solve this by blasting the e-coil and then being more careful about venting more fresh air, and then leaving it set on fresh air when parking. Opinions? Thanks.