Another thought. Can you insert extra fuses after F15 but as close as possible to where the wiring splits off for the circuit? Or is access too difficult?
Say F15 powers 5 circuits. Insert smaller fuses (10A?) as close as possible to F15 but in each of the 5 circuits.
If only F15 blows it is in the wiring before any of the other fuses or you have missed a sixth circuit.
If any of the other fuses blows it is that circuit.
It could help to isolate the problem by eliminating "good" circuits.
A complication is the fuses are in series with the larger F15 but if you set F15 to 30A and the others to 10A it is likely the 10A will blow before F15 blows.
I have been trying to think of a way of using the return current to the battery (it must be over 20A) to identify the region in the bodywork it flows through but I think and voltage drop it would be too small to be useful.
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F15 powers about 20 circuits. That's why it's been so tricky to debug. But yes, I have tried a version of your idea, and will try more once the gearbox is in (hopefully Friday)
So far I've isolated the dash/instrument binnacle, and powered it of it's own fuse. At least that means what when F15 blows I don't lose speedo, eng temp, rev counter and fuel gauge which was threatening to get me a speeding ticket! And F15 does still blow, so that rules out the instrument cluster as being the fault.
At the moment, the headlight levelling ECU, glove-box, sunroof and rear heated seats are unplugged - so not them.
I have previously unplugged the level control sensor plug (OSR trailling arm), radio, climate control panel, MID, underbonnet fusebox fan, Cruise Control, courtesy lights, drivers door window control switches, and light switch. F15 has continued to blow with these things disconnected.
Therefore there is either something else taking it's power from F15 that I'm not aware of, or it's a short in the wiring. As far as I can tell, all the 'body' systems get their F15 power via connector X1-pin49. All of the 'engine bay' systems get their F15 power via connector X5-pin1.
The plan at the moment (once the gearbox and exhaust are back on) is to locate these connectors and try to isolate these pins. Ideally I'd like to pull the pins out of the connector, but I doubt I'll have the correct pin extraction tools. So if that fails, much as I dislike wiring bodges, I'll have to snip the wires close to the connectors and bodge-wire in fuses - probably 5A. Then if either of new 5 A fuse blows, I'll know the fault is downsteam of either the 'body fuse' or 'engine bay fuse'. If F15 still blows then it's pointing towards the wiring in the dash.
I don't think there is much point speculating on the cause now until I know which of the 3 fuses blows.