How do I get rid of it? I'm guessing it's moisture build up but man it started to really get smelly.

 

how many miles do you have on the car? how long have you had the car? you might want to change your in cabin air filter.

 

There is a few ways you could fight it. First, try replacing the cabin air filter. Then find a can of frigi-fresh and spray it into the interior air duct intake through the cowl near the wipers. The others may not be practical but you could remove the blower motor and try cleaning inside the ducts. And you can also try getting to the bare heater core and evaporator. The evaporator is actually the source of the smell. All the moisture and bacteria settles on it. The frigi-fresh and cabin filter should do the trick. Although, I can't really stand the smell of frigi-fresh, but I guess it's better than that aweful stinch!

 

There is roughly 1800 miles and I always use the a/c. It's been on recycle mode so I just read in another thread that putting it onto the fresh air cycle may help.

 

Keeping it on recycle causes the smell to happen faster. Putting it on fresh now without treating it most likely will not help much. At the very least, the smell will be there on cold starts and go away after about 1 minute of fresh air use.