Rear side marker lights wont work

Ok , so if you have the correct bulb and 12 volts going into its center contact, to path to ground is through the side of the bulb base to the bulb socket. The bulb socket must ground to the fixture and so on to the car body. Too much paint or a socket not tight in a potmetal fixture will loose the ground.
Frankly, I'll peel those old metal bulb sockets out in a heartbeat. The little white rubber sockets for #158 bulbs will fit in there and provide 2 wires. I can then go straight to ground or make them blink sequential if I choose to. This also makes replacing a bulb a heck of a lot easier on some models/fixtures.