I cleaned out my cowel vents through the heater box removal and the drivers cool air vent removal ,,,used a tooth brush bent with a heat gun ,,,,to clean around the steel fange and other area`s,,, even taped a stick to it to get the hard to reach spots,,, and a vacuumed it all out with just the hose only ,,,,,,,,,,,,so it could be bent to s u c k close to the egde of the flange......Bu t first i power washed all i could get out from the top side of the cowel vents before i started on the inside ..........Also if you have a hole by the flange for the heater box,,make a new flange to fit over the old one,, and flare the bottom by cutting slits in the metal so that the bottom of the flange its larger than the rusted area apply sealer around the flange bottom where it meets the vent,,spot weld or rivet the flared bottom......to the cowel area and apply sealer over that as well....