I can not remember Kendals last name, but he installed most every thing himself when it came to electrical. I beleive he even installed his own tank. These tanks are really nice, made sturdy and will take a bang really well. Your best bet is to talk to Kendal if you can get ahold of him. He really had his own way of doing stuff. We got our tanks at the same time and christo and kendal did the electrical in a unique way and I did it a real easy way (with no circuit board). I beleive we all spiced into the level reading on the dash guage the same way. On the driver side under the dash down by the pedals to the left above the dead pedal you may see a wire spliced(aftermarket) into another wire(stock not super big bundel). this is more than likely the dash wire feed to the fuel guage. There usually was with this install a little switch to the left of the steering wheel (black small square, up and down position not marked). This switch controls what tank you are viewing on the dash guage. the switch with the circuit box controled the fuel pump and the automatic like functions of the stock system that is in Aus.
To find what wires kendal used at the back with the stock wiring diagram you will need the toyota Electrical Wiring Diagram book for the specfic model year you are working on. In most cases the EWD is close enough to work for a couple of model.
The switch you are showning in the picture is a stock toyota sub tank switch for you model year truck(and several years of range). The plug on the end was the way Kendal connected the wires to the truck and the control unit. If you did not trace and pull this control unit you may be SOL on this as the boxes were a custom make and christo may or may not Make another. You would have to contact him. The sender in the aux tank is toyota part and has toyota specfic resistance to match the dash requirement in the 80 series. The fuel pumps we used for this was some from Carquest, and it strictly was a transfer pump(one that could be run dry). it pumps fuel from the aux to the main at about a gal a minute, There also should be an inline fuel filter that you may want to change out at the time you swap. You you need to talk to me you may reach me on my cell 303-884-7813. I no longer work for christo. later robbie