On ALL trucks in '76 and '77, whether single OR dual tanks, the fuel gauge wire (tan) comes from the main bulkhead connector at the firewall & routes through the engine wiring harness: it follows the starter wires through the heat shield tube by the RH exhaust manifold, then branches off & is clipped on the inside of the RH frame rail & ends at the RH fuel tank. Wire has the round rubber "push-on" connector. This wire is the same length on all trucks because it's part of the engine wiring harness -- GM did this to minimize the number of different engine wiring harnesses they had to make.
This tan wire plugs into the RH tank sender on 1-tank trucks. On 2-tank trucks it plugs into the dual-tank wiring harness instead.
The dual-tank harness plugs into a round grommet in the firewall just to the right of the distributor cap area (if you have a V8) This harness goes straight down & follows the LH frame rail.
The blue wire exits the harness first & attaches to the the LH tank sending unit.
The harness crosses over to the RH side & ends close to the selector valve. The green wire goes to the selector valve.
The tan/white wire attaches to the RH tank sending unit.
The tan wire attaches to the fuel gage wire discussed above.
It sounds weird but yeah, that tan circuit does make one big loop around the whole front half of the truck. If you are wiring from scratch you can cheat & just run the tan wire from the back of the fuel gage directly to the tan wire on the fuel tank selector switch.
The switch sends power through the green wire to the selector valve when it's at the LH position.. so if the unit goes bad, you will be pulling from the RH tank.
Dual-tank wiring setups changed several times from '73 to '87 so this only applies to the earlier trucks. Not sure when the changeovers occurred.
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