First. What year is your MJ? 87-90 used really crappy ford sockets that have the worst ground tab in the world that has probably rusted out by now causing your indicators on the cluster to illuminate as they indicate a ground fault in the circuit.
As for the flashing marker lights, if I recall correctly, the ground for that light to flash is shared somewhere in the turn signal circuit and I think it uses the filament for the flash as a ground until the flash turns on and thus the marker shuts off and turns on opposite to the turn signal.