Turn signals don’t work when headlights are on

"How this works".

A poor ground creates a series circuit. Normally the tail power goes to the tail filament, the stop lamp power goes to the stop/ turn filament, and they are both grounded through the shell

If the lamp comes ungrounded, ......................

With the tail lamps off, all tail and park filaments are in parallel to ground. This creates a fairly low resistance circuit, and if the shell is ungrounded, all four tail filaments provide the series ground

If you tromp on the brake, or turn on the signal, these four tail/ park filaments carry the stop lamp power from the "shell" end of the stop lamp filament through all four tail filaments to ground.

This "works" until you turn on the park/ headlamps. Now the tail lamps are powered, and the ungrounded shell no longer grounds either the tail filament OR the stop/ turn filament WHEN THE STOP LAMPS or signals are on, they work "opposite." That is, with the brake unapplied, NOW the stop/ turn lamp filaments are acting as ground for the TAIL lamp that is ungrounded