If it is coolant from the heater core, you should be able to smell a sweet odor when you run the heater. You can taste it also, it should be sweet. It could be the AC drain plugged also. The AC runs with the defroster to keep the windows clear so there is condensation even in the winter.
chek the passenger side for water under the carpet
The most prosaic explanation is that something spilled there and the water soaked through the carpet.
Moisture accumulating on the air conditioning evaporator, often due to mold buildup.
your heater core is leaking
Plugged evaporator drain?
Blocked condensate drain hose on bottom of evaporator.
It is your a/c condencer ot should be draining out of the truck. the rubber line is more that likely clogged.
check sunroof for leakage
HEARTER CORE STARTING TO LEAK .PASSENGER SIDE FLOORBOARD UPPER TOP CORNER. LEAK WILL START TO GET CARPET WET AND WINDOWS FOGGED UP.
I hasd a similar problem with a 1990 Camaro. My carpet was wet from a leaking heater core. Smell the carpet where it is wet. Does it smell like antifreeze or is it the color of antifreeze? If so nyou have a bad or leaking heater core. Good Luck
better check your heater core.
If your vehicle has a sunroof, the drain tube may have popped out of it's outlet. This tube runs down inside the roof support on the passenger side of the windshield, running down just in front of the passenger door behind some plastic. I have a 2003 XRS. It had a really musty smell that we traced to the front passenger side carpet. When I pulled the carpet back, it was dripping wet. The air conditioner drain hose had pulled away from the firewall and was draining into the floor pan. I replaced the tube into the hole and have not had any trouble since.