The ET igniton has 2 distinct and separate power supplies
They then share a single common earth path
Up to now, I have deliberately avoided mentioning "earth" (or 'ground'), because a) Rick is a relative newbie and b) ET electrics are AC so there isn't any "earth" (no DC distinct 'supply' and 'return').
Moreover, if there really were "2 distinct and separate power supplies", the ignition would only work when the brake pedal was depressed, so the lamp was on ...
So well done for clarifying that ...
If the taillight lights
And the brake light does not ,
It is not the original common fender/frame
... that is/was at fault
Mmmm ... especially when the brake filament draws three times as much power as the tail filament and we have no idea of the condition of the bike and the components that make up the "fender/frame path" ...