Where my A/C ductwork was leaking I could place metal duct tape over the leaks with a 3' dowel rod. It isn't pretty, but doesn't leak.
I had to remove the metal plate attached at the right of the motor access door and coolant overflow bottle.
I've read old posts were they were able to loosen the rear cap but leave the top seal intact, pull it open enough for a slim person to slide up between cap and back wall to re-tape the ductwork.
I doubt that would work if the ductwork is broken/leaking near the top though. Somewhere back years ago a member posted pictures of removing the rear cap and patching the ductwork. As it remember the cap was very heavy and hard to handle using multiple people.
2000 Winnebago Ultimate Freedom USQ40JD,
ISC 8.3 Cummins 350, Spartan MM Chassis. USA IN 1SG 11B5MX,Infantry retired;Good Sam Life member,FMCA." My fellow Americans, ask not what your country can do for you, ask what you can do for your country. John F. Kennedy