It sounds like you replaced a lot of things that may not have been the problem. Diagnosis is always good.
Try cleaning the key resistor contacts with a rubber pencil eraser-both sides. Then dip the key in alcohol and insert quickly into the ignition to carry some alcohol to the contacts there that touch the two sides of the resistor. They may clean up a little. The contacts wear due to friction. So you may be past the point.
You can try turning the key to ON and watching the security light. It should come on for 5 seconds or so while it reads the key resistance, then the light should go off. If it starts flashing, most likely it'* not reading the resistance. I tried inserting key to ON, then pausing, then turning to crank to give time for readings to get done and the theft deterrent module to decode properly.
The wires from the key cylinder run down the steering column and the fine little wires start breaking due to tilting of the column each time someone gets in or out of the car.
Bypass the VATS system by putting in a resistor set in series to give the right resistance. Find the VATS info on the net about the 15 key resistances. Put together with solder a pack of resistors from radio shack to give within 10% of that resistance. At the bottom of the steering column the two fine wires from the lock cylinder come down to a connector with many wires in it. The two little wires probably are inside and orange tube which looks like it'* a single wire. They split out to two contacts in the multiconnector however. I ahve seen pictures of this connector on the net; but different years and cars may have different connectors.
Once you plug the resistor pack in there or cut the wires and connect the resistors to them (making a circuit that goes to the connector and toward the TDM which is above the glovebox on the right side) the car always reads the right resistance when you turn the key. However any copy of your key will work even if the resistor is wrong, so theft deterrence is lowered.
Or I have read on the net, you search youself, that if people cut the resistor wires down at the b ottom of the column whihle the motor is running the TDM goes into a failure mode so that it doesn't check for resistance when key is turned to start.
The TDM also may go into failure mode just from having the resistance come and go while driving. That'* what my 98 did. I get no light, never. It just isn't working. If it does ever start again, I have 4 resistors from Radio Shack (.99 cents) that I can plug in at the bottom. They total 880 ohms which is my key reading almost exactly.
Dual auto air on both