How do I know if the heater core is bad or if I have other issues-Water leaks

What is your question?

Problems have been discovered within the last 2 weeks Could there just be a water leak. It seams worse when it is raining. The windshield is covered with moisture in the mornings and frosts on the inside of the windows especially the windshield. The car smells like mold. There is water on the floor and there is mold on the floor mats. The car wasn't heating up on the drive from Athens Georgia to Cullman, Alabama (4 1/2 hour drive) But the car didn't overheat.

What makes this problem better or worse?

When it's raining-cold outside

How long has your 2012 GMC Terrain had this problem?

2-3 weeks