The outside front marker lights blink for turning also on my 79. The parking and turn signal bulb socket has a funny ground wire on it. Mine was corroded. You can take the bulb out and turn on the lights to see if the indicator light comes on. Then check the ground.
You don’t need a relay. Hook your marker light wires to the turn signal hot and the parking light hot. When the turn signal is on the marker gets its ground through the parking light filament and flashes the same as the turn signal. When the lights are on the marker gets its ground through the turn signal filament so when you turn on the turn signal it has 2 hots going to the marker and shuts off. When the turn signal blinks off the ground comes back and the marker lights. Thus the alternating on and off. Hope I explained it ok.