1994 Ducati Super Sport 900 CR
Yessir agreed that is the starting point 👌I said it earlier but I guess I should say it again: there are two wires that go to the tail light assembly. One wire is to the brighter brake light and one is to the dimmer tail light. If the tail light wire is directed to the brake light, the brake light will illuminate when the bike is turned on. When you apply the brake, the tail light bulb will illuminate but because it's dimmer than the brake light, it's hard to see.
I would investigate that first before doing anything else.