someotherguy
Even more awesome in person
I sure did seem to recall the ground attached to the bed support was a white wire, but without a working example I couldn't confirm off my truck, since I've just got the black wire there currently. It was badly hacked by PO's, and I've undone about 90% of the hacking. My taillights, signals, brake lights, etc. all work but haven't got the hazard flashers on board yet. (Flasher was melted, replaced it assuming old damage due to previous hacking, still don't work.)What's nice is that both the black & white wires are included in the G401 wiring diagram.
Referencing the factory manual is always a wise choice but I do get OP's desire to confirm from real-world. That almost always makes more sense to my brain than wiring diagrams which tend to make my eyes cross. However, that's the official version, not 25+ years of whatever has happened to these poor trucks once they left the factory.
Richard