[Richard]

Tapping the running light circuit seems logical. Add up wattage of all roof marker bulbs. Watts/volts=amps. If worried about added load just put roof markers on a relay and power the relay with the running light circuit. Tapping at the marker may prove easier if you do not want to trace the circuit.
Edit- You may be able to put your HL switch in the park lamp position and check for continuity between red wire and others. That should show which wire is hot for the park/running light circuit.