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Could be a couple of things
After engine is warmed up and idling, set temp inside to HOT and pop open the hood, feel BOTH heater hoses at the fire wall, both should be warm/hot which tells you the heater core inside the cab it circulating coolant.
If one hose is much cooler then either the Valve on the heater hose(you will see it) is not operating or the heater core is plugged up/blocked.
If heater hoses are both hot then your Blend Door is not working.
Google: Ford Ranger Blend Door
In 1995 they changed from cable operated to electric motor for Blend door operation
And these electric motors fail or break the door's tab off.
It is located behind and above the glove box inside the cab, hard to get at
The Blend door diverts air flow away from heater core(COLD) or thru heater core(HOT), AC evaporator always has air flow, to dry the air for better Defrost.