Here's how bad grounds "can" "sortof" work
Front and rear park/ tail/ stop/ turn lamps are a two filament deal, with two contacts. Each contact is one of the two filaments, and both filaments are grounded to the bulb shell
Here's what happens if the socket / shell is not grounded:
With the park/ tail lights OFF, and you hit the turn signal, sometimes the lamp will light and blink, because the circuit path goes........from hot side turn filment contact, through the turn filament, through the ungrounded connection between the two, through the tail lamp filament, and then over to the other three park/ tail bulbs AND TO GROUND through all three bulbs
With the park/ tail lights ON, the offending socket has power now on the park / tail lamp, and no ground. So THAT goes from the hot contact through the tail filament.........through the ungrounded shell........through the stop/ turn filament, and TO GROUND through the stop lamps (if at the rear) or NOT TO GROUND if on the front because the front is not hooked to anything else. It MAY light up the dash indicator lamp
When you activate front turn with a bad front socket, with the tail / park lights on, there is 12V on BOTH sides of the lamp and no ground, so no current flows
In the rear, with tail/ park on, and turn signal selected, same deal basically
To try and put this more simply you have created a series circuit. From 12V through one filament, through the other filament, and off to the other tail / park lamps, then to ground through them.
Confused? Just check your sockets LOL
