[ACK]

From what you're describing, you have the thrift air heater. If Pimpton65's diagnosis isn't the issue, here's some additional thoughts:

If you're barely feeling a breeze coming out the defrost or bottom heating vents, good chance there's a rodent or mud wasp nest in the heater box. Disconnect the heater control cable and unbolt the heater box inside the cab and see if there's any blockage.

If air is blasting out, but it's not hot, some culprits include:
- Air pocket in the heater core
- The hoses on the heater core are swapped
- Thermostat stuck open

Good luck and let us know the outcome.