I had the same problem. Bought an OBP-2 used and when I got it, could not figure out what extra wires were for. The seller had capped each extra wire
separately with a wire nut so I left them alone (just bundled them in). Lo and behold it worked (sure surprised me)!
I was connecting to a single (P-Bass) pickup too.
I'd advise just ignoring the extras and see if it works.
Good luck.
