Hello ratsun community, I'm sure this has been covered already but I'm not finding what I need to know. My 79 620 k.cab's heater isn't fully functioning.The defrost nozzle/vents for the windshield work, and blow heat.The Rt./Lt. Vents ,close to the doors do nothing, no heat and no air flow.Finally, the vent(s) below the dash (center) , where you push the know in for vent Does work, however NO heat ,even though I've got heat Uptop through the windshield vents.I am pretty sure that in the past ,the vent below dash (center), when pushed in I would have heat there as well..Could this be do to a resistor or perhaps the heater core itself?..I am contemplating removing the whole heater assembly to inspect, clean,lubricate cabling and or refelt , blah blah ect.Mabye I'm imaging heat coming from the lower vent, it's been a while and the side vent never worked.This 620 is NOT A/C equipped..Thank you wise ones..