2017 peterbuilt 579 vents not blowing air, defrost barely on

Agreeing with deer hunter . Old vehicles had a slide lever with a rugged cable to change from ac to heat. New stuff have a vent control( keeps hot air off the heater core from mixing with your ac). POS part (plastic) may be electronic with a little "door" on it. May or may not be a pain in the arse to get to (replace)