Your battery should have a big ole heavy strap/braid going to ground, and a big ole heavy wire going to the top of the starter.
When you hit the foot switch, you're completing the circuit to the starter motor, so if you have a good battery, and solid battery connections, the starter should crank regardless of the ignition key-switch position.
First thing i'd do if I were you would be to disconnect the smaller wire from the top of the starter, so the only thing you've got going to the starter is the hot side of the battery.
If the battery is good, and the starter still won't crank, and you're ABSOLUTELY SURE YOUR BATTERY CABLES ARE GOOD, AND HAVE GOOD CONNECTIONS ON BOTH ENDS.. then i'd say next you would wanna check the actual starter switch.
With the battery disconnected, take the two screws out that hold the switch in place.. this is just a spring loaded hunk of metal that makes connection between the terminal and a bare nub on the starter. This area tends to get corroded and full of crud. I use a toothbrush sized wire brush and some WD40 to scrub 'em out good.
If you removed the starter all together at this point, you could hook some jumper cables to a battery, clamping the ground to the mounting flange of the starter, then the hot cable to that "nub".. the starter outta spin.. if it dosen't, either you arent following the directions right, or your starter is toast.
If the starter behaves normally with the rest of the truck wiring removed from the circuit, it could be you have a wiring SNAFU somewhere down the line thats sucking your battery dry, and thats why the truck won't crank.
