I’m confused by your description of the gauge wiring. I assume you have the stock switch and wiring for a ‘76, where the solenoid had a single power terminal.
There should be one tan wire coming from the gauge cluster, and it runs through the firewall, down the passenger side frame rail to the tank on that side. If you only had one tank it would just plug into the sending unit there.
The wiring for the second tank takes a very roundabout route.
When they add a second tank they install the dash switch, and run four wires over to a grommet that goes through the firewall below and to the passenger side of the brake booster.up cod ‘76
- The wiring runs down the drivers side frame rail, with one wire coming out and connecting to the tank sending unit on the drivers side.
- The other 3 wires cross under the truck with the metal fuel lines and connect to the passenger side sending unit, the wire coming from the gauge, and the power terminal on the solenoid on the passenger side.
The DPDT switch on dash has five wires.
First half
- power from fuse box on center terminal
- power wire to solenoid on one side
- open terminal
This half of the switch determines if the solenoid has power. With no power it defaults to the passenger side tank.
2nd half
- wire that runs to the gauge on center terminal ( but remember the complicated routing)
- sending unit wire for left tank
-sending unit wire for right tank
This half switches the sending unit wire from left to right tank.