or to explain it for you... the bulb has 2 filaments with 3 connections they have separate positives but use the same negative.... when the tail lights were off and the brake lights were on the current traveled through one filament to the shared negative, back through the other filament, through the tail light wiring and through all the tail lights and markers on the trailer and the vehicle to their good ground.... it caused the tail filament to glow in the light with the bad ground but since the current was then shared bu all the others in parallel it was not enough to make them glow..... when you turned on the tail lights that (limited) ground path became positive and since the bad light now had two positives and no ground it went out
