Originally Posted by
Simon Barden
With 4 wire pickups, the wire soldered to the shield is the ground wire, so those should be soldered to the back of a pot.
The two other wires soldered together (red and white) join the two pickup coils (that make up the humbucker) together and can be used to split the coil for single-coil operation (with a suitable extra switch) or are left unused for standard humbucker operation. They should be insulated with some tape to prevent then shorting out to ground on something.
The black wire will be the signal wire which goes to the volume pot.
The factories change pickup supplier from time to time, and the wire colours change (there is no universal standard). The pickups used can also change from single wire (plus shield) to 4-wire (plus shield) connections and back. So it's hard to produce a single diagram that always matches the pickups supplied.