Good news plus more good news.
GN#1:
The converter box is the problem. I had proper response from the test light on the wires on the 5 wire side of the box, but only the taillights on the output side.
GN#2
The local Napa store has the converter box for $15. It is in stock, and one of them is in “will call” right now. That’s way better than $60ish for a new complete harness.
I could wish for a little more slack in the wires I need to attach to, but I can probably do what I need.
GN#3
That means I think I’ll be able to avoid pulling out the platform or opening the interior panels.
Thanks to all who responded and answered questions pointing me in the right direction to identify the problem and know how to fix it.
I am grateful.
–Roadtripper