I posted exactly the same question some years ago on the other forum.
Never found anything and tracing the wires was a waste of time too.
Eventually smoke started to pour from the roof lining and the cables feeding the unit all fused together.
Needless to say it is all isolated and I removed the unit to make yet another little storage cubby. Check the wiring above the unit and where it disappears into the roof lining just in case you have the same issue
What I did find was, there is a little gyro unit in the middle of the boot roof lining that connects to it and a lot of the cables went down to the original double din radio unit. This had been ditched before I got the car so I don't know if it was integral to the compass etc.
I imagine the compass and gyro were part of an original sat nav unit.
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