no heat from vents

I have an 80 Monte Carlo.I'm having problem with no heat coming from my vent, but I do have heat from the dash. Will someone please help me. Thanks.

I think this year used a cable operated vent control. They also had a vacuum controlled valve to shut off the hot water to the heater.
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This was the type on my 1979 Impala. The parts store sells vacuum hose which you can cut 1 11/2 inch piece and push on any broken plastic lines.