We have been experiencing a lot of heater/defrost issues in the last few years. These issues are occurring on our newer trucks that have multiple doors in the dash that direct air and heat to different locations in the dash. We have found most problems stemming from foreign objects falling into the defrost ducting. (Pens, pencils, air freshener sticks etc.) This repair can be quite extensive, sometimes taking the dash almost completely out.
So please, make sure not to have anything that can fall into any of the vents to prevent ducting issues.
LANCE