It appears to me that this is the OBP-1 Preamp, the 2-band boost only. I'm just looking at the wiring colors, so correct me if I'm wrong on that, it's a bit of a rat's nest in there and hard to see in 2-D (no offense to you or the wirer, it's just how preamps can get sometimes, I know all too well!).
Here's the factory wiring diagram:
And this is the added bypass switch wiring:
It looks like the switch is right, from your drawing. And It also appears that the jack is right, assuming the RING connection only goes into the battery box. The red from the preamp should also be connected to the red of the battery box and nowhere else.
I've had some issues in the past, personally, where the ground connection seemed to "be better" for some reason on another spot. In other words, what seemed logical about grounding everything to the same spot wasn't as effective as running certain grounds to other grounded points. Not sure why, but it's happened.
If this were me, I'd basically be starting a rewire from scratch and do what I could to clean it up a bit. But I honestly enjoy that sort of activity, YMMV.
I'm not seeing an obvious problem with the diagrams (though maybe the coffee hasn't kicked in just yet), so there may be something that is unseen causing the issue. All it could take is the ring lug contacting the shielding to complete the circuit. That said, just rewiring the jack with shrink tubing on the exposed lugs is a good place to start.
And as for the volume, here's what the test would be:
- If you pull it and the pan stops working as a pan, and there's a lot more OOMPH :it's a series parallel switch for both pickups.
- If you pull it and the pan works and the pickups sound different, it's likely more of a "coil tap" for the stacked pickup setup.
Again, hard to tell by looking at it from the angles, but that's an easy way to see what it's doing.
