are you saying that when you hook the test light from the sender to ground that when the light comes on the gauge also registers? if that's happening then the sender has to be bad, or else you've got a bad connection at the sender or you've used some thread sealer [or teflon tape] on the sender so it's insulated from ground
if there's power through the gauge to the sender, the gauge can't be blown, but it may be that you got those gauge connections shorted out, make sure the cardboard piece is insulating the pegs from the case - normally when you first turn on the key the gauge will jump at least slightly up off the peg [cold]
if you look at the factory schematic, the position of the wires is indicated by location on the diagram, note that the layout is just like in the truck and there is a line on the diagram labeled "front of dash", top is right side [ign sw], bottom is left [lite sw], and it shows the sender wire [grn] closest to the ign sw, and hot wire ["ign"] to the lite sw side, so you seemingly do have them reversed
if you short the green wire to ground at the sender, the gauge should read full hot, but you might try that with the wires reversed from your picture
Bill
