Have any tips for tracking that down?
Ratty harness or not, it is getting the voltage there. With the game PCB unplugged, I get +5.2v (on the +5REG line) all the way to the pin of the edge connector. When I plug the game PCB in, I get +4.27v, measured either on the trace on the game PCB or on the molex on the AR2.
So something seems whacked in the game PCB.