Im installing a 6v LED Taillight (from Benchmark Works). Its a two function light that is as simple as it gets. Low Intensity for Taillight and High Intensity for The Stoplight Function. Bench test is perfect. From the terminal block behind the Battery the wires through the fender to the Taillight test fine with a 6v source connected.

With the bike wiring connected to the terminal block the Taillight function is on high intensity so that the Brake light will not work. (As the light is already on high intensity).
If you disconnect the tail light function, the brake light does work. With the key in the run only position, the brake light does work.

All of the above is without the engine running and a new 6v Battery.

History is I must have checked for Brake light w/o the lights (taillight) and assumed it worked with the (incandescent) taillight also. Not sure now.
Also the previous owner did rewire the headlight and mentioned the 4 tangs that hold the ignition plate. The spare key, w/o the plastic housing, is the only one that works now. I absolutely do not want to take the ignition plate out and disturbing the tangs. Especially since l do not know how many times it has been done.
I did successfully disassemble and reassemble one in the early 70s and the nub on my shoulders has been ground down and the fingers dont do what they are told nowadays.

Im stumped as to why the bike wiring seems to have the taillight on both functions.

Any suggestions will be more than greatly appreciated as What I know about Electricity will
Shock you.