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...

Thanks for pointing this out, @howardr. I don't think I said to randomly solder a wire to another wire. I was just saying to look for clues to where the wire might have disconnected. I should have explained this better in the previous post. Sometimes a wire can get disconnected and it will leave a solder indentation, which might match up to the wire that became disconnected... But, the best way to troubleshoot this would be with the machine's schematic. Hopefully, someone will post information about the Grand Slam schematic...