First off, STOP RUNNING IT UNTIL YOU FIX IT! I've had a couple in for repair where the coupler shorted out and took out wiring and melted the connector. One also melted the mating connector on the PS/3 board sufficiently that the board had to be replaced!
Start at the coupler and just make sure the wiring from the coupler to the PCB connector isn't shorted, pinched, etc. It's a straight shot going through one 2-pin connector, so it should be fairly easy to trace.