I've seen "hacked" wiring like in your photo many times before - typically loom slit open and a hideous combination of solder, tape and Scotchloks as the result of an alarm having been fitted. Your bike has almost certainly had an alarm fitted at some point, which has given grief (as they usually do) and has been removed. What you've been left with is several exposed terminals (in the connector shown) which lead to the bikes circuits. Any moisture, corrosion or abrasion can then give trouble if it bridges any of these now unused terminals. As well as this there will be various points where the wiring has been tapped into which will be weak points. I take it that the white multi-pin connector is not usually plugged into anything?

Usually the circuits that are used for an alarm connection are: L/R indicators, brake light feed, backup circuit for instruments (fuse #6), imobiliser and earth. Any bridge between two or more will allow some current to flow and funny things to happen, maybe not always 12V as the bridge will usually have a fairly high resistance - it won't be a good connection, hence the 8V sneaking through to the kill switch.

I think by pulling fuse #4 all your doing is removing the symptoms, rather than isolating the cause as your cutting the power to the ECU?

Try pulling fuse #1 (lights, brake lights, horn) as this (brown wire) is the usually used for an alarm's power supply. Any spurious current will likely be coming from this source. I reckon your disco lights will stop. This will also explain why the fault goes when you apply the brake (light) as the current in the brown wire can flow to ground through the brake lights and will be reduced enough in the bridge/short that's causing the problem for the symptoms to disappear, but the fault is still there.

Let me know the results or pulling fuse #1.

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