The blend door has a foam seal that tends to fall apart. The result is that the blend door won't completely seal out the cold air inlet and the air from the heater won't be as hot as it should be. It won't cause a complete lack of heat.
With a complete lack of heat there is no coolant flow through the heater core for some reason. It could be air in the cooling system, a plugged heater core or a bad coolant pump. The original pumps had plastic impellers which also tended to disintegrate. Since you have the original pump still, this could well be your problem. Even more reason to do that timing belt and coolant pump right away!
I think I fixed my no heat problem. I changed the thermostat but was still not getting any heat. I turned the car on turned the fan speed off and turned the air temp to hot. Then pinched the top radiator hose until I felt a lot of pressure build up and let go. I turned on the fan to 2 and noticed I was getting a little bit of heat. I did that a couple more times after car warmed up and now I have excellent heat lol. I don't know if my heater core was plugged or not but this worked for me and hope it's not a temporary fix. I drove over 300 miles in it today and didn't have any heat issues. On the other hand I have been having some oil leaking previously. So I changed the two gaskets on the oil cap but still had the leak . Then I changed the valve cover gasket so far no oil leaks. Will be changing the water pump and timing belt in the next week or two since it is the original ones on there still.