Neutral / park switch if the unit has automatic trans.
Clutch depressed switch if manual transmission.
simple test. Run a wire from the positive battery post to the alternator output *that red wire on the back of the alternator). Try starting the Jeep. If it starts you've blown a fuse link. If not you've got other problems. With the Key on in the Run position does ANYTHING work?? Radio, Gauges???
Just because an item has been Replaced does not ensure that it is NOT Defective. No Power to any electrical with the Key on is a Prime example of a Wiring problem. Usually a Fusible Link on the Starter. Fusible Links look like a wire and may look fine to the eye however, like a fuse be burnt under the protective coat. This is why I asked if you have power with the key on to your accesories. If NO power simply run a wire to the Alt from the Pos on the battery and leave the key and radio on. If the radio starts as soon as the battery is connected then you have a Blown Fuse Link. Skipping steps lead you astray.
If you have power to the accesories then is it an Automatic Trans or Stick? If it's a stick check tyhe clutch Depression Switch on the top of the Clutch Pedal. Hint it's that plastic thing that has 2 wires and moves with the Clutch Pedal. If it Automatic, wiggle the shifter in Neutral while holding the key on the start position. This checks the Neutral Safty Switch.