What car are we working on? Sounds like you need to get a proper wiring diagram for it anyway. When you say "at that juncture" do you mean where it branches to the dimmer or at the dimmer end. If at the dimmer end then something seems wrong as all those wires should be blue, two with tracers. If at the branch, would it be under the dash? If so, I can't think what it might be unless it is the other end of the same G/P wire at the brake light switch. If so it should eventually go to the brake lights. Do you have a meter to check the continuity for that?