I had a Ranger of the same make and year, my brother still owns it.
The water pump gave up the ghost as I pulled into the parking lot at work one morning. So I took it to a repair shop just yards from the factory to get it replaced.
The heat always worked great before the change, really good, I had the truck three years, after the new pump it never worked right.
I gave the truck to my brother shortly after.
Over the years he has replaced the water pump, thermostate and it has a new heater core, he ever replaced the hoses. He still complains about no heat! I agree, it takes forever getting the air out of this system.
If you figure it out, please be sure and post about it, I will let him know. He still drives the truck to work everyday, he hates it in the winter!
He has an electric defrost unit he plugs in to keep the windows clean in the coolest weather.
BTW: You might try your question here:
I never asked about this problem after I gave him the truck, but they where a help with a couple of other issues.