Usually, if the ground for the parking light lamp is not grounded at the socket, then, when the lights are turned on, current flows through the turn signal filament and back to the corresponding instrument panel turn signal lamp.
You can see in my diagram that if power from the headlight switch (BRN wire) can't go to ground at the parking light, it is rerouted through the LT BLU or DK BLU wire to the T/S indicator and finally finds a ground.
