89 camaro

my son has a 89 camaro convertible.the heat has stopped working.thought it was the thermostat but that wasn't it.does anyone have any suggestions on what could be wrong.thanks:

Check the inlet and outlet hoses to the heater core and see if the inlet is hot and the outlet is warm,also check the hot water valve in the inlet side. These usually start to leak, get plugged up or the vacuum line comes disconnected.