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After engine is warmed up and heat control turned to HOT, let engine idle and open the hood
Feel BOTH heater hoses at the firewall
Both should be HOT, one only slightly cooler, slightly
If one is much cooler then heater core is plugged up, no circulation, so must be replaced, not and easy task in 2000

If both are cooler then By-pass valve is not working, the thing with the 4 heater hoses attached, it has a vacuum valve, make sure vacuum hose is attached, Grey vacuum hose
Try to move valve arm manually to see if it is stuck, heater hoses should warm up if valve opens.
If valve seems to work then check vacuum hose it should have vacuum with cab heat set to HOT, if not then trace vacuum hose and look for the break, it runs into firewall.


If both heater hoses were HOT then Google: Ford Ranger Blend Door repair
Loads of videos
Blend door is inside the cab and routes air flow around or thru heater core, so if it is stuck closed then no heat, no air flow thru the heater core


If you think you have air in the system then with engine COLD
Remove rad cap, it will not be filled to the top if there is air in the system
Remove 1 heater hose at the firewall.
Top up rad, a bit of coolant should flow out hose and heater core when at the top of rad
Reconnect heater hose
Put rad cap back on
Pull off overflow from rad cap opening
Blow into it until you hear bubbling from overflow tank, it could be blocked, which stops coolant from coming back into radiator as engine cools down
Replace over flow hose
Drive it a few days, then when engine is cold check coolant level, should be at the very very top, no air at all
If there is air then you may have small coolant leak