Toyota Repair: Toyota celica Gt, toyota celica, air doors

Question
I have been having problems with my heating on my 1989 celica gt  it will start out with heat but then after a few minutes it has no heat i have tried replacing the thermostat and also replaced the control cable but still no luck

Answer
Whic control cable was replaced, the one for the heater valve or one for the air doors inside under the dash? Has the heater valve under the hood been checked? It may have to be replaced. Even though the valve moves with the cable it can still have a problem inside, if everything else is ok then there may be a problem with the heater core, it could be partially blocked with rust, check the coolant in the radiator, if there is a lot of rust the system may be able to be flushed but if too severe the heater core may need to be replaced.