Outer Limits 4x4 Board

hi need some help i have a gq 4.2 deisel ute and i have 2 tanks which i think is after market setup.there is a switch in the car that switches the tanks over and also switches the fuel gauge over on the dash i traced the wires back from the switch and they 2 run to the battery and 2 run to this pump looking thing and it seems to be getting power to it what is this thing it has all the fuel lines running to it has me stumped has power what could it be any help would be good cause i just filled the tank

thanks