There's a vacuum-actuated valve on the passenger side firewall that controls the coolant flow to the heater. The vacuum is turned on and off with a vacuum switch controlled by the climate control module. This switch goes bad after many years of service. You can see if the valve is opening by starting the car and seeing if the lever on the valve moves. If it doesn't, route the vacuum line past the small switch and straight to the vacuum port to see if the lever moves then. I needed to do this to my car 3 years ago, and I still have the switch by-passed and the heater working fine.
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