At this rate you'll have cut all the wires before I get home from work. Have you ever worked in bomb disposal David? That was very much tongue in cheek by the way... please don't be offended :)
I concur that the yellow wire(s) are not involved, being the SCP network bus. I was thinking about a different wiring concern... ooops.
I also agree that the correct wire according to the diagram is a white / red to FC942.
Using your photo for reference, it is the wire on the BOTTOM row, second position in from the RIGHT. That traces back to the ignition switch accessory function. If that is the wire you've cut then it should isolate the ignition controlled supply to the audio module. So I'm now at a loss as to why the radio still switches on.
I think at this stage, I'd test for B+ on FC942 with the ignition off, then accessories, then on, just to test the function of the ignition switch. I would then keep checking FC942 with ignition OFF over a period of time. It should not ever become live with the ignition off, even after the periods of time you've mentioned. If I'm right, that would, in turn, suggest an intermittent B+ within the audio module.
Further, if the ignition switch has produced B+ when switched off, all the other accessory circuits should be live as well. Have you checked that David? Figure 01.4 ( Ignition Switched Power Distribution: I (Accessory) ) will tell you what pother circuits are live on accessory.
BTW, soldering or barrel crimps for re-joining. These are power supply wires so you don't want to take any risks. PLEASE do not use Skotchloks.