There should be 3 wires on the back of the switch
1 - from the front tank
1 - from the rear tank
1 - to the gauge (probably the center one)
Easiest way would probably be - turn off the rig, have the switch set on either the front or the rear tank, unplug/disconnect one of the outer edge wires on the switch, turn back on power to see if the gauge moves - if it does than you know which tank it is reading - if it doesn't move, flip the switch to the other side and it is for the other tank. If it doesn't move on either selection than you know you disconnected the wire going to the gauge. Using the above method you should be able to sort out pretty quick which wires go where. Once you sort out which wire goes to the gauge and which goes to the rear tank, at that point splice, solder and join to your hearts content. Make sure that you properly seal the other wire so that you don't have it wandering and completing a circuit sometime later down the road. As for a wiring diagram, I am sure that someone has some, but if the PO changed something, you might be SOL. You could also use a MultiMeter to figure out which goes where, but that is more work than just a simple disconnect to figure out.