IF YOU WANT ALL OF THE LIGHT HEADS TO BE SYNCHRONIZED(not alternating)
You must do the following:
Tie GRAY wire from one side of the vehicle, to the WHITE wire of the lightheads on the other side
and the YELLOW sync wires need to be tied together as well!
If you want them to be ALTERNATING (theory, not tested yet - but should work)
Gray wires will need to be tied together as the 2nd flashers
White wires will need to be tied together to be the PRIMARY flashers
YELLOW WIRES ALWAYS NEED TO BE TIED TOGETHER