Oh, that foam - I ripped mine out while repairing the seams where firewall meets floorpans, which rotted courtesy of the sound insulation Ford put on the engine bay side of firewall... And yeah, from the pics now I can kinda figure out you got the vinyl floor, mine looked very different in the rust department... That insulation also went in the trash.
Anyways, it ain't your heater core leaking, I suspect it's the seal between the plenum and the firewall. Or the grommet where that harness goes thru the plenum. Or the foam where heater core ports go thru firewall. I'd pull the hoses (cut & replace if so needed), trim some of the excess foam around the area and see where there may be a gap. I actually have several wires ran next to that harness, I sealed them to plenum w/ RTV and not a drop of water inside. So I'd recommend spreading some grey RTV over that foamy mess (after trimming it). And pull up that insulation piece on inside and look under it for holes in firewall, IIRC there's at least one in there that should have a plug thru it, but may be missing it.
Also, you do know we have a PM system, right? Don't just wait and hope on a particular someone who you think may be able to help you to log in, you can send PM, I bet very few folks will "yell" at you for it.