My first thought is to check the harness and that the wiring is all in order. Refer to the manual for the colour code and make sure it is connected correctly and with nothing touching where it shouldn't.   Any unused wires should be isolated so they don't cause issues. For example, the unused gun LED + wire will still carry a voltage when the trigger is pressed, so best to check it isn't shorting on anything.   A variation on Bertus' suggestion would be to disconnect all wires apart from the power feed + and - and test from there. Reconnect  the other wires one by one and test each time. If/when the fuse blows, you'll have a better idea of which wire/feature is causing the problem.

@Dalek gamma I will, would be a shame to pass up on a rare Curse Supreme opportunity

Hello,   I see the power also goes over the plug. You can not power the unit without the plug. You can measure the isolation between the wires of the plug with a multimeter with ohms setting.   Bertus

Not sure on that one. Only ever removed it when the unit was switched off. Can try that tomorrow.