Its more than likely your heater core.
If you have lost a sufficient amount of coolant this will stop the heater working because there will be no hot water in the heater rad.
Could be a hole in a hose, a heater hose, the water pump could be leaking, there could be a hole in the radiator. Many times condensation from the AC evaporator is mistaken for a coolant leak.
heater hoses. or could be the run off water from your A/C unit
Your heater core, which is under the dash is supplied by coolant from the engine. If it is leaking, or the hoses to it are leaking, it could end up on your carpet. Also, if the drain for the A/C evaporator is clogged, you would have a similar result. This sometimes happens after getting a car undercoated.
Leaking heater core
You just have to see where it is leaking. There are numberous gaskets and hoses it can be coming from. Have it preasurized and it will be easier to find.
Plugged A/C condenser drain line if it is plain water. If it is coolant, then the heater core is leaking. Another possibility is a leaking windshield.
if it has rear heat--it has hoses and a heater core for the rear heating system--could be any of this
If it is leaking under the truck it is not the heater core that is leaking. It would leak inside the vehicle on the passenger side. There is a drain tube for the A/C that is under the vehicle on the passenger side, and water from that with the A/C on is normal. If it is coolant, check the heater hose connections at the firewall. If you find that the heater core is actually leaking, replace it. No other repair is permanant.
could be leaking from a head gasket and trickling down or your heater core is leaking and driping out of the condensation drain from inside the cab. most likely its your heater core. check the fire wall above the starter and look for that drain line. if it is dripping and is colored (green or orange) its coolant and coming from heater core. that's a bear to replace. good luck.
it is possibly a heater hose
Heater core, windshield, or a clogged cowl drain line. If the liquid tastes sweet it is coolant and the heater core is leaking. Heater core must be replaced. If not, it is one of the other 2 problems.