I need to know where the radiator which serves the heating system is
The heater core on your F150 is buried in the middle of the air distribution box (plenum) near the the blower fan, behind the dash. Not exactly easy to access. I suggest having a technician, like one from YourMechanic, come out to you and diagnose your A/C system to pinpoint the leak and perform the repairs.
I don't know how cold winter gets in your neck of the woods, but you have another option. You could loop the heater hoses in the engine compartment to bypass the heater core and do without heat.
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