So looks like I have to replace my heater core

I was taught my method by my mentor..

At the Dealership we get paid flat rate 1.3 hrs, and I do them in 0.8,, that's
an extra .5 each job, and adds up on the pay-check..

My concern would be that if the box has never been out, that the j-hook is seized,, it'll cause him a problem trying to get it out and may consequently break the housing, or mangle the hook making the job much more difficult..

I'm also concerned that the vent hoses usually tear as they're often brittle/rotten.

And often that foam ring at the cold air inlet will shred/tear/disintigrate,, and time will be wasted trying to find/repair that seal, when you don't have any replacement materials readily avail..

Then he has to figure out the cable setting, which could be challenging for a novice..

As my mentor used to say.. "Shoot youself"..

Sorry Tony . . .. jmo

hope it helps