Optocoupler U1 on interconnect board. Diode test the optocoupler. It's probably bad. Wild guess based on the information provided.

http://www.pinwiki.com/wiki/index.php?title=Williams_System_9_-_11#A.2FC_solenoid.2Fflasher_problems

This got me when I was building my solenoid test board for my bench rig. The issue is that the system is distributed across multiple boards using multiple components. It's NOT obvious.

If you have a DMM you can follow the instructions @ http://www.pinwiki.com/wiki/index.php?title=Williams_System_9_-_11#4N25_Opto_Couplers_in_the_Switch_Matrix and see if the component is bad.

I was thinking why Williams added this (in Jokerz! - after Taxi) and the reason dawned on me. It's because if the relay fails then attempting to power the flasher with a failed relay will fire the solenoid instead of the flasher and the game will behave as if it were possessed. It would still work because the solenoids are on the A side whereas initial Data East games reversed the A/C and a failure of the A/C relay would simply power the flashers and no solenoids would fire making the game a boat anchor. The 4N25 optocoupler allows the software to verify the relay engaged the flasher power. If the relay doesn't engage the software doesn't ground the drive transistor because that would ground the solenoid and not the flasher (in the A/C relay failure case) - i.e. your game is not possessed.