If the coolant level in the reservoir is not going down, we can rule out a leaking heater core. In addition, the leaking coolant would drip onto the ground and you would smell a sweet smell. If the smell is musty or moldy, the AC condensate drain is plugged and water is collecting in the drain pan due to a blocked discharge tube. Look under the hood on the passenger side of the firewall for a 4" rubber hose hanging down. Squeeze it to open up the end. If no debris comes out, squeeze the wire ring and pull the tube off, then carefully stick a pencil up the spout to clear it.