Wouldn't the gauges work like normal if the wiring is not messed with?
Negative, there is no physical connection between the senders and the switch in the dash - it's the tank selector that moves the contacts to choose which of the sender gets connected to the gauge. That said, there's nothing stopping a determined person from hooking them up to the factory switch, it's just quite a bit messy to explain how to do it, cause the switch has two wires that loop on the back of its connector. Instead I suggest extending the senders an gauge wires into the cab, and using a simple SPDT (3-pole) On-On (no middle off position) switch to do the gauge switching between senders. Running off brick wire colors, you want the yellow/white (gauge) wire from the TSV on the middle pole of your new switch, and the darkblue/yellow (front sender) and yellow/lightblue (rear sender) wires from TSV on the two end poles of the switch. Fill up one tank to the top, keep the other tanks at half, and try the switch: if the gauge shows the wrong tank for the position of the switch just flip the two sender wires around on the switch poles (gauge wire always stays in the middle tho), and if one of your tanks reads empty then you have a loose connection between switch and respective tank sender.
As for the factory switch, you can just unplug it from the dash harness and leave it be, or you can use it to power something you put in the bed, or you can just replace your heater controls w/ one for a Bronco and do away w/ switch altogether.
So just hook the gauge wires from the switch to the wires coming from the tanks, and viola. I think.
Negative, there is only one gauge wire but two sender wires. A switch is needed to connect them properly, unless one wants to add a second fuel gauge. The factory switch is not connected to the gauge or the senders, you can either make the connections yourself (not quite as easy as you think) or run a separate switch.
