Most single coils have two ends on the super thin wire that is the coil and each will be terminated to a larger piece of wire that can be handled and soldered. This could be any sort of wire, but it is usually different colors but of the same gauge, like 24. Additionally, there is often a third piece of wire, often mesh such as that which shield cables, which is soldered to the body of the pu to ground it. This can be combined with one end of the coil and they can be run together as the ground, but often they are not and instead the cable used is 2 conductors inside a shielding. If every thing is shielded it works out cleanly, but w/o complete shielding it is possible to get a ground loop which will make annoying noise and can be very frustrating to run down. In your case just wire up the red as the hot and the mesh as the ground. If it doesn't work you may have to dig for a missing wire, a task I rarely succeed at. I am much better at the wooden parts.