1993 Soveriegn - O2 sensor wiring - tracing

Looking for help tracing what seems to be a wiring fault in the oxygen sensor circuit, diagnostics so far below:

Car lost power during a journey with "Fuel Fail 44" on VCM, limped home to find that this is the code for the Oxygen sensor, so out with Haynes to check:-

1) Sensor heater resistance 4.8 ohms - haynes says should be "about 5-6 ohms"
2) No voltage on heater circuit at pigtail connector
3) Oxygen sensor relay not switching - relay coil is between white/brown & pink/brown wires
4) White/brown wire has 12V when ignition on; pink/brown wire shows no voltage with ignition on or off.
5) Neither White/brown or pink/brown have continuity to earth with ignition on or off

So, it looks as if the relay should be switched by the pink/brown wire going to earth, but this isn't happening. At this point the Haynes wiring diagram ceases to be helpful, just shows pink/brown going to a "high power board".

The wires disapear through the bulkhead, can anyone tell me how to find this high power board without stripping the entire dash?

tia

JonS