You don't have to unpick the loom. Just remove the petrol tank and trace the loom back from the clocks to the ECU looking inside each connector on the way and checking that it (the loom) isn't worn through at any point. You basically need to do a visual check first (nothing more technical) just to make sure there is no physical damage, corroded connections, loose connectors or anything obvious that is causing the problem before delving deeper.

I'll see if I can find and annotate a diagram/photo...

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