I had a somewhat similar problem with a car's headlights. In both a high-low headlight and the usual two-filament stop/turn/tail bulb, there are two filaments inside the bulb, each having their own connection plus sharing one more connection. Interesting circuits can be made of this.
In my Boler tail lights
(and presumably in Kevin's Burro), the shared connection is the "ground" (white wire) - that's the wall of the socket (or shell of the bulb base). The two contacts on the bottom are for the two filaments. In my car, the common connection was broken, and when one filament was supposed to be on, power flowed through the filament which was supposed to be on, then "the other way" through the one which was supposed to be off, across to the light
on the other side of the vehicle, through the other one which was supposed to be off, then back to "ground" there.
This drawing shows two bulbs, their corresponding contacts connected together, a bad ground on the left bulb, and two paths for power when only one is supposed to be on: one path (shown in green) lights
up the intended filament of the right bulb, the other (shown in red) goes through all the other filaments, lighting
them up dimly (because they are in series). This is what happened with my headlights, and shows a possible way to have light
in a bulb with no ground connection. Maybe nothing to do with Kevin's tail lights
, but maybe a possibility to consider.
Edit note: I realized that the wiring above was for my headlights (as I mentioned) but the bulbs looked like tail lights (which have only one common circuit, not two), so I clarified the drawing.