Sounds like this is going to be an "extra" wire from the bulkhead connector down the left frame rail since both tanks have their own wires? I believe there is already a wire to the gauge from the switch.
Look at the diagram in your original post...
The fuel gauge tan wire (Ckt 30) at the top of the diagram is the wire you found "cut and hanging" along the right frame rail. In other words, that tan wire is the wire used by the factory to connect the fuel tank meter(s) to the fuel gauge, regardless of whether the truck was equipped with single or dual tanks. Unless someone previously altered the factory wiring, the fuel gauge is not "redundantly" connected directly to the fuel tank selector switch through an additional dash wire.
With dual tanks, the factory fuel gauge wire (tan) ran down one frame rail and connected to the dual tank harness (tan, or in some cases orange), which then ran back up the opposite frame rail into the cab and selector switch. From there the fuel gauge wire split through the selector switch then ran back down the frame rail to each fuel tank sending unit via light blue (Ckt 30B) and tan/white stripe (Ckt 30C).
Since you're in the process of rewiring (unless you need to duplicate the factory harness for judging purposes at truck shows), you can eliminate the 18-gauge (Ckt 30 and Ckt 30A) tan (and/or orange) wires along both frame rails. Then, reroute the (Ckt 30) tan wire running between the instrument cluster harness plug and the bulkhead connector directly to the selector switch without exiting the cab. That way you will have only three wires running out to the left tank (light blue), right tank (tan/white), and selector valve (light green).
Has this helped clarify it?
Yes !
thanks for the explanation,
we were scratching our heads trying to figure out where all the wires went for the gas gauge.
we had a half dozen wires for the gauge that should only have one.
we bought a pickup with a 80 radiator support, (according to the smog sticker) has apparently a 77 wiring harness according to the colors, with pre 76 doors. dual tanks, but only one hooked up to the engine and no selector switch, and the tan wire that runs down the passenger frame rail was hooked to a terminal on the starter???
we added TBI so we're not sure how to hook up the dual tanks, but we will figure it out.
1
