89 fleetwood. only getting cold air from vent
 

I have an 89 fleetwood with the 4.5L v-8. The issue I'm having is with the heater. It blows but I'm not getting any hot air out no matter how long it runs for. I have already tested the heater core and put a new heater valve on it(heater valve was not present on car when bought). thermostat is working. The guy I got the car from said something about if the heater would not blow hot air you can turn off the heat and turn it back on a time or two it would start working properly. Not sure how to test heater controls to determine if the issue is there. Any help would be greatly appreciated my grandsons have to ride in this car.