Referencing the first photo:
This setup is apparently a balance, master volume, 3 band EQ with no active/passive switching, so it's always active. The master volume is after the preamp output so it is probably a lower than normal resistance, like 50 K Ohms or 25 K Ohms.
There's no "hot" signal wire coming from the blend control! There's the black ground wire that's soldered to the other pots, no signal wire. Most likely it's the brown disconnected wire is the preamp input but I'd try to find some more info if possible. But it appears that everything else is connected, so that's the odd wire out.
Secondly, the positive battery lead where the red wire is soldered to the purple wire...that needs to be insulated ASAP and before the electronics are re-installed, Heat shrink is ideal but requires cutting and resoldering the connection. Just wrap a little bit of electrical tape around the connection. You don't want the 9 volt battery wire touching the shielding or any pot terminals. It could kill your battery quickly and possibly release the magic smoke.