Re: Soldering an extra wire to 1C pickup??
Single conductor means 2 wires, hot and ground, and that's all you need for phase between two PU's (works for HB or single coil). Phase is just switching lugs between hot and ground. To do this properly with P-90's however, there should be an extra ground (3 wires in total). If not, you'll get more hum.
FYI: so that you know in the future for other PU's, I add a wire to convert single conductor HB so that I can have coil cut or spin-a-split. Easy to do. Take off the PU cover, and peel back the tape at the end covering the wire connections. There's a wire for hot, one for ground, and the other two are connected to each other (between the coils), which links the coils electronically (these would normally be the red and white wires on Duncan's 4 conductor HB's). Take the tape off the solder holding them together, and add a shielded wire to that connection, solder it in, run it down to the control cavity, and that's the wire used for coil cut or spin-a-split.