So, with key ON, you should have power at the fuse.
You should also have a 2-pin Radio connector under the dash with one terminal having two wires, White and Red TR. It should have power with key on.
You should also have power at the White wire at the switch down at the shifter or transmission.
When the trans is in reverse, the switch should be closed, allowing power through to the Violet wire.

That Black wire that feeds the fuse comes from the ignition switch and also feeds the wiper switch. It should have power with the key ON or ACC.

Kind of cute how they tie the radio into the reverse lights. That way, you know something is wrong with the reverse lights.
Same sort of thing they did with the cluster lights. No power to the headlights? No power to the cluster lights either!

BTW, there have been stories about wiring harnesses that are not all wired correctly.
Hopefully, this will give you the info needed to trace the circuits properly.