Quoted from fredsmythson:

It's probably a wire that shares a switch lug with another wire. Look for a switch lug that has bright solder, which might show a broken, shared solder joint. Also, other Gottlieb machines of the same era might use the same wiring color for the score motor switches. So, using other available schematics might help with the wire location...

Quoted from pinhead52:

Is the game functional/playing? If we knew what was broke you can then find the lead on the diagram. or somebody that has this game.