OK, take the plug off the switch and check for continuity from the plug to the marker lights. If you have no continuity you know the problem is a break in the circuit. If you do have continuity check the pins at the switch for voltage. If you have voltage at the switch where the plug connects the marker lights and continuity from the switch to the marker light it has to be the bulb itself or ground. You could take the plug off the switch and apply a known 12 volt source to the marker light terminal in the plug and see if the lights activate. You have power to the switch to light up the headlights. The same switch lights up the markers, if there is no juice from the switch to the harness plug, it has to be the switch. If you're getting juice out of the switch to the marker plug but no juice to the bulbs it has to be a break in the circuit. Strange to get a break in the front and the rear circuits at the same time though.
