Quoted from MarkG:

There's a schematic on IPDB.org.
[quoted image]
The orange-black wire (in the red box) connects a switch on the 100 point relay to the 9th position switch on the 100 point score reel on each of the four players. Your loose wire is probably missing from a solder tab on one of those five switches.
The 9th position switch closes when the score reel shows a 9.
/Mark