If there is some heat but not like it should be then my guess would be the heater core is getting plugged up. Try pulling the two hoses and back flushing it. put some radiator cleaner or clr in it and let it set a bit. Then flush it out both ways a few times. A good idea to do anyways, whether or not it fixes it. Then make sure your coolant is topped off and is bled of any air. Air trapped in the system can cause insufficient heat as well. Good luck. Let us know when you get it figured out.

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1996 Ford Ranger
Ext Cab XLT
2.3 L SOHC
145,000 miles