Was this ever resolved? I have the exact same problem, but I traced the issue to the 9-pin Molex connector that connects the Reels opto harness to the main wire harness just to the right of the slot assembly under the playfield.

Easiest way I was able to test this was to put the machine in reel test, lift the PF up, disconnect the motor plug on the right slot reel PCB, and then manually turn the right reel until the indexing notch in the reel is aligned with the opto. Once i did this, I set the reel test to right reel. Since the motor is disconnected, the reel DOES NOT spin, and therefore allows you to mess with things until the opto reads "closed" you'll hear the beep when it closes.

It was driving me crazy until i grabbed that 9-pin molex and wiggled it, thus activating the opto.

Ha, turns out molex was just fine- it was a wire that had broken free of it's solder on a target switch just above the slots on the PF.

These pins are very stressful sometimes! Makes the feeling of fixing something even better!