Check that the heat control cable actually works the blend door in the heater enclosure for m the control lever.. If that's OK, then check to see that the heater hoses are both gettiing hot. If only one gets hot, the heater core's pluigged. You may be able to flush it. or you may need to replace it. last, be sure the engine's getting up to temperature, it should get to about 195F degrees warmed up. If not, it may need a thermostat. Use a 195F degree stat.