Hi Susan, I've been mirroring most of your leak stop efforts in my own 83 wagon.
I just took a quick look in mine for the hose you are describing but I may have been looking in the wrong area.
From the engine bay side, passenger side, I see a wiring harness up high, just below that is the AC hose to the evaporator, and off to the right (towards centerline of car) is the small vacuum hose for the HVAC controls. These all poke through the black metal "insert" in the firewall.
I don't see the yellow/translucent hose or the one you are describing, so maybe they are further outboard and hidden by the strut tower sheet metal. I can't get a clean look at that area from inside because I have an amp mounted on the inside of the firewall up there.
That yellow hose sounds like the tailgate washer hose. It should connect to the sprayer to the side of the tailgate.

I'm wondering if the cut-hose side that you describe was never a hose at all, but is just the back of a mushroom-shaped plug. I would look around for a hardware store or parts outlet that has a decent variety of rubber plugs in the neighborhood of 1/2" I.D. and wait for a dry day to pull it out from the inside and replace, cleaning around the hole after removal and using silicone, RTV, or urethane sealant.

Joe