1973 Dodge Dart Sport No Heat.

Checking the heater core with a garden hose is easy to do but for now let's assume that is not the issue.
Here is a list of possible problems.
Make sure there is vacuum at the hose connected to the heater control valve located under the hood on the right inner fender.
Bad vacuum actuator under dash or vacuum hose has come off one of the actuators.
Malfunctioning AC Heater control switch on dash.
Other members may have additional ideas.