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Over the last three days, I took on the task of sorting out the universal 12‑circuit Speedway wiring harness to adapt it to our truck—and what a nightmare it turned out to be.

On day one, I nearly gave up altogether. I seriously considered splicing back in the old rewiring job I did a few years ago and selling this harness for whatever I could get. Returning it wasn’t an option, since I purchased it almost a year ago.

The real issue is that this harness wasn’t designed for a slightly modified truck like ours. Instead, it’s clearly intended for a retro‑mod build with a modern engine, transmission, and a GM steering column that incorporates the turn signal and ignition switch. In fact, they even supply plug‑and‑play connectors for that exact application. If someone has that setup, this harness would be ideal. It includes circuits for an electric fan, electric temperature and oil gauges, electric wipers, hazards, and an electric choke—none of which I currently use.

The harness comes with twelve circuits total: six constant‑power and six switched‑power, along with two relays (one constant and one switched). I ultimately used five constant circuits and one switched circuit, with additional circuits reserved for a future radio and AC/heat system.

Before cutting anything and potentially rendering the harness useless to anyone else, I needed to be absolutely certain it could work. So, painfully, I labeled every single wire. Yes, the wires are labeled every twelve inches, but I can’t read that print without extra light and stronger cheater glasses. Using blue painter’s tape and a Sharpie, I marked each wire with its circuit number, where it came from, and where it was going. I also opened the back of the fuse block to determine which circuits were constant power and which were switched.

With that information in hand, I confirmed that the harness could be adapted—but only with significant modification. Still sticking to my “no cutting until I’m sure” rule, I mapped out the fuse block, reassigned each circuit, and relabeled every wire accordingly. I also identified which wires would be eliminated, which would be coiled up, and which would remain in the loom for possible future use.

On day three, I finally reconstructed the harness into its new configuration and drew a detailed diagram of the fuse block and wiring layout. I plan to add each component (HEI, Headlight relays) and their respective wiring to the diagram so I’ll have a complete and accurate reference going forward.

In the photos below, the first shows what the harness comes with and the second is my diagram of the way I reconfigured it.

Now it’s on to installing everything back into the truck and, hopefully, getting it running again.
Wish me luck.