try changing out the heater core or the therma stat may be bad
Check that the switch that moves from cool to hot (Is like a sliding switch on a 1990 Corolla I think) is actually opening the vent that controls flow of hot air
The heater isn't an extra unit, it is simply pulling heat from the engine and blowing it into the cab. If the engine is not hot the heat will seem like it is not working. If the heat is not working after the engine has warmed up, you may have a fan that doesn't work, or a clogged vent.
heat and a heavy hammer the larger the better
Check the heat shield on the catalytic converter
Have you checked you heater control valve or heater core if working properly.
Have you flushed the heater core?
No. The problem is most likely your heater core. Ask a mechanic.
the heater of your car does not only work when the car is in motion. it only works when the engine is started. you don't have a heater in your car like you do your home. the heat that you feel when you turn on your heater in your car is produced by the engine. and there for if your car is not on then you will not receive any heat. some cars take longer than others to "heat up".
Do you have heat when you turn the fan on?
make sure the flipper opens all the way around the heater core
BLEND DOOR FAILING TO MOVE TO OPEN UP HEATER CORE ACCESS.
i have a 93 accord with the same issue. i found that the control valve for the heater core was stuck close. i just opened it by hand and now i have heat