What makes a fuel guage read backwards? In other words, if the tank is low it reads full and vice/versa.

Check for a bad ground at the sending unit at the tank. If there is a bad ground on the sending unit, it will cause the gauge to read full.

a bad ground at the sender, or an open wire to the sender will cause the guage to read way past full all the time, it won't move or read backwards.

'Bout the only thing that will make it read backwards is to hook up the power and ground reversed. Is this a factory guage?