windshield constantly fogged steaming hot inside vehicle even with ac on, seems to be the heater core needing replacement, it's a costly repair.

I've changed the thermostat and radiator fluid. The temperature gauge doesn't reach the middle either. I am in the u.s.s army in germany and it gets cold here. the vehicle sat for 8 months while I deployed to kosovo, would that have an impact. somebody mentioned a heater core, could that be it. thanks