To find the correct wire, turn on the headlights and use a 12V probe to find a power wire.
Double check it to see if there is power when the lights are off and if there is power when the headlights are on.
What Buggman said is the correct way to do the wiring.
Marker Lights and Parking Lights are the same wiring but the Marker is tapped off the Parking Light circuit.
Look for the Parking Light wire instead of the marker light, it is a little larger gauge.
Run that wire to pin 86 on the relay and that should activate the relay when the parking lights are turned on.
Use a fuse in-line, low amperage rated, to be on the safe side.