Side vents are cool air only. The inlet tubes at times may fill with debris. If no warm air to heater/ defroster, check the water control valve for proper adjustment or malfunction.
cheers, Jon

 

Drive car, air should come out of side vents. They are cold/no air only.
Check if the temp slider cable is attached to the valve that lets hot water into heater core. If the cable is attached and moves the valve things work correctly. If no heat, either the heater core is plugged, valve does not function correctly or the hose from the manifold is not connected to the heater valve.
Top slider all the way left is max air. Bottom slider all the way right is max heat.
Turn on fan switch. With door on heater assembly closed (right behind shifter boot, can be difficult to open), air comes out defrost vents. Door open, air comes into cabin.

 

On carbureted engines there is a air purge valve on the intake manifold. Check for collapsed inlet water hoses. Rubber diaphragm in water valve sometimes malfunctions. You can adjust the valve throw by loosening the cable, opening the valve fully , and retightening the valve set screw?
cheers, Jon

 

I’ll take a look at this further. Thanks for all the info folks!