My AC was performing poorly (little cooling) & thought I lost freon. This forum discussed duct leaks, so I investigated & found a leak at the rear duct, between the rear cap and the rear wall. I took this photo lying on my back & the rear cap is to the right. The leak is at the bulge:
The duct breach is about half way up the rear cap, and about 3 to 4 feet past my extended arm reach. Removing the cap rang $$$ in my eyes, so came up with another repair method. Using metal duct tape (backing peeled off), I tacked the top to a 1"x3" four foot board as in below photo:
Then w/flashlite in hand, lying on my back, carefully put the board up and against the breached duct seam. Repeated the process 4 times to ensure coverage. BTW I first cleaned the area with a rag (attached to board) with some alcohol. No I did not have a shot myself.
The finished product is not aesthetically pleasing, but got the job done.
Anyway it worked. I submit this as an alternate to removing the rear cap, in those cases where the duct is reasonably reachable. Above that level, the cap may have to be removed. If the duct is physically separated, that's another story. An alternative might be a very thin grandchild with a penchant for RV repairs.