Air conditioning condensor in the vw vanagon fell down what is the sequence to put it back up?

If this is a non-camper model, you would start at the rear unit and work your way forward. There was a recall on the AC ducts on some models because the plastic would crumble around the screw heads that held them up. My forward duct section (between the driver and passenger seats) once fell and had to be put back up. This required removal of the front ductwork over the windshield as well, and that piece has to be installed last, as it holds up the front part of the forward longitudinal duct. If the plastic is all cracked to pieces where the screws were it's not going to be easy to repair. You may have to find good used units as replacements (and that won't be easy).