Okay, if you're not getting power when in the on position, I'd trace the wires coming out of the switch and look for a split/short. If you cant find anything it might be worth it to buy a new switch. The center post coming from the switch is where the main power goes. Put your wiper switch in the on position, maybe spay some water on the windshield if you like. Run a test light from the battery and tap it into each hole in the rubber plug you pulled from the ign switch until you get the wipers to work. If they kick in then I'm gonna say its the switch that's bad. If they don't, then something else is wrong with the wiring. There should be two plug holes that will actuate the wipers, "acc" and "on" wires. I didn't want to teach you how to hotwire the car, but try this. If you tap power to one of the wires on and it engages the starter then I think the closest hole to that one will be the "on" position. So, run a lead from the battery to the "on" wire and tap another lead from that onto the "start" hole you found earlier. That should crank the car and start it. You should also hear the fuel pump with the lead to the "on" hole.
Just thought of this, check the fuel pump, fuses, fuel level...ah, I see you have EFI. You might want to try the 5.0 forum too, just tell them you have an 89GT or something to keep it simple. They might have ideas about the computer etc. I hope you figure it out.