1973 Dodge Dart Sport No Heat.

Thats not a manual valve, its vacuum operated. Never mind the two things in the cowl. Those are 73 only knox sensors I believe and usually run to the distributor. You have two vac lines coming out of the firewall to the left of the heater hoses. One runs to the heater control valve and the other to the vacuum amplifier (round black sphere on the left). Make sure the vacuum amplifier has vacuum on it. It should have a larger hose that probably runs to the intake or the base of the carb. Also, manually try operating the heater control valve. It may just be stuck.