91 Sundance heating problems

Not holding heat? I guess you mean you have none? What do the heater hoses feel like going in and back out?

You probably didn't bleed the air out of the cooling system after changing the thermostat. That's what the threaded plug atop the "water box" is for (the "water box" being the part of the cylinder head where the upper rad hose connects, where the thermostat is located). On a '91 with 161K miles, you may find the plug seized in place, in which case loosen the radiator cap so it holds no pressure, elevate the front of the car as high as practicable, and run the engine at a fast idle with the heat on until normal engine temperature is reached, then for another five minutes or so. This will allow air to travel to the radiator and out to atmosphere.