I have some bill Lawrence single coils that have three wires. one that is soldered to the base plate of the pickup and two others that attach to each opposite end of the coil. Which is the ground or how do I determine it?
The base plate is definitely to be connected to ground. In practice, either one of the other two, but not both will be connected to ground as well. It would only matter which if you have more than one pickup connected, as that would determine if they will be in phase or not. Out of phase sounds very thin. Sometimes desirable on a guitar, but on a bass? Don't think it's a good idea, but hey, you never know!