Heater blowing cold air

Make sure the radiator is full, then after the engine warms up and the thermostat opens, feel both heater hoses under the hood at the firewall where they go into the heater core. They should both be hot. If one is hot and the other cooler, than the heater core is clogged and you could first try a back flush. If both heater hoses are cool, see if there is a working heater control valve on either heater hose on the engine and if so, that the hose is hot on both sides of that valve. If hot before but cool after, see if the valve is stuck or a vacuum hose fell off the valve.
If though both heater hoses are hot at the firewall, then the problem would have to be with the distribution ducts under the dash and most likely a blend door that is not operating, so if so, check why it the blend door is not moving, perhaps a faulty heater control, a broken cable or vacuum hose that fell off the blend door.

Are the heater hoses hot at operating temperature?

Let's make sure you have the

1) Blower on (hear it running)?

2) Air knob pulled out till it stops (this is the half way position).

(2A General information) Beings this old, if you pull the air knob harder (and your pal assists the "frozen/dirty" vent Lever against the passenger wall). The knob will project further out, this would be the air or vent position being fully on, (air from the vent on the cowl). Summer time air 'twixt your legs and on the passenger's legs. Many folks do not realize the half or full positions, they think a quarter of the way is the heat setting.

3) Temperature knob all the way out?

4) defrost off/or part way/to fully. This will rob the heat going to the floor if pulled fully on.

(Note) after the air knob gets used to the full position (WD-40 helps too on the linkages and cables) It will function without your pal's assistance, just will a firm pull past the half mark.

A) Heat mode/ no defrost.

B) Air knob at 100% see "C".

C) This vent will be open if air is at 100%/ It will be closed at first stop/ the half position for heat mode.

D) Twixt your legs air/vent/ duct under the steering column.

E) "Mr. Jeep".

The Medic