2009 F250 here . Got to where had no air flow also. Found a video showing how to drop glove box and cut a 6 inch long by 3 inch tall door in plastic in front of evaporator. Only cut top and sides of rectangle. Fold the now formed flap down and clean with water . Water will run out of drain tube if you don't spray too much at a time. I use a radiator cleaning wand. After you get it clean, simply fold flap back up and cover with duct tape. Future cleanings will be a 10 minute job.