66 dart electrical problems

OK the socket must be ground.

The bulb must be correct. There are several numbers but two are the 1034 and 1157

What these essentially are is TWO bulbs in one. One contact feeds a small low wattage filament through the ground shell for tail and park

The second contact feeds a larger higher wattage filament for turn and stop

"Things" happen. Socket shell not grounded. Wrong bulb or bad bulb. Sometimes the bulbs can throw you for a loop. IF the internal interconnect for the ground shell comes loose, you now have two filaments in series across the contacts, which can give weird results

Probably worse is when the two contacts internally become shorted together. This an also happen in the socket