Are you the original owner ? Are your heater hoses hot to the touch ? Is the hot water valve opening ? Have someone cycle the heat temperature selection on the dash controls and watch to see if the hot water valve control arm is moving. Or pull the vacuum hose off of the valve and see if it moves to the open position. But first, feel the hoses to make sure they are in fact getting hot water through them.
If you're not the original owner, a previous owner may have unplugged the rear temperature control blend door actuator to keep it from "clicking" after it's gears broke. You can lift up that rear tray in the rear quarter panel to inspect it and see if it is unplugged. Or the actuator may have broken a long time ago and quit clicking already. But the rear heater core gets hot water continuously, there is no control valve for the rear. Just the temperature blend door in the rear box.
Rear blend door actuators. Green circled is high/low vent outlets. Red is temperature actuator.
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