My 98 Expedition 5.4 had the heater core bypassed.
Seeing a heater core bypassed you should assume that was done because the core leaks.
The more you dig into that rusty old Expedition, the more problems you are finding. I told you that this would happen Fitz. Good luck replacing the heater core, I think that the dash has to be removed to get to it.
I have already replaced the heater core...twice.
A huge PITA. Less of a PITA the 2nd time, since I knew the procedure, replaced all the cracked plastic dash parts while I was at it. Dash did have to come out . First time I cheaped out and used an aftermarket core. It got a pinhole leak at the tube. That is when it was bypassed by a mechanic. 2nd time was OEM and I just guessed which was inlet and outlet side. Since I am replacing the hose, I wanted to make sure I install it in the right position while it is easily accessible.
BTW, my Expedition is definitely old, but only a few rusty parts remain. Over the years, I have done minor body work, parts replacements and painting. A little at a time is easier for me financially.