[patrickt]

First step, is to get two ten foot long wires, of typical lamp cord gauge, with alligator clips securely on the end of each. Clamp one end of each to the negative terminal of your battery and the other end of each to the black wires directly at the connection to each of your rear light assemblies on both sides of the car. Yes, I know, you're 100% sure your grounds back there are absolutely pristine, but just do it anyway, and re-test. We will leave those ground wires in place throughout the diagnostics.