On the chance that I need to create my own good common grounding point for the pioneer, empeg, speakerphone and/or OEM amp, does anyone have a recommended point somewhere within a feet or two of the OEM amp and the center console position?

-brendan

There should be a factory ground point up by the factory amp located in the passenger kick panel. You should see a bunch of ground wires tied together and bolted to the sheetmetal. The ground from the radio harness should also terminate at this point, so you shouldn't have any problems using the ground wire from the factory harness. As for grounding another amp, just make sure the ground wire is as short as possible. There's no need to run it straight to the battery or a common ground as long as you follow the rules for running signal and power wires.