I'm guessing you have a 4.0 I-6. Mine is a '92. The good news is that it is easier to find a no-crank problem than a no-start-on-crank problem.
My starter is on the passenger side of the motor, towards the bottom of the block, just to be sure we are on the same page. My starter is actually the combination of two devices: a motor and a solenoid. The solenoid is the smaller cylinder at the top of the assembly. The solenoid has three electrical contacts: two large ones and one small one. The large one that is closest the starter motor has a large wire that goes from it to inside the motor. The other large contact is connected via a large cable to battery + (a terminal post arrangement nearby the battery with a lead to the battery + terminal). The small electrical connection is the control terminal.
This system works like this: voltage applied to the control terminal causes the solenoid to close, connecting the large terminals together within the solenoid. Current from the battery flows accross this connection and into the starter motor windings, and into the starter motor housing, into the block, and back to the battery via a large wire from the block to battery (-) post. The starter motor is energized and cranks the engine.
Tricks to find the problem:
Verify that the block is connected via a large wire to battery (-).
Turn the headlights on. Verify they are hot and bright. Have some attempt to start the vehicle. If the headlights dim significantly then the battery may have high internal resistance. Check posts, electrolyte level and other features of the battery. If the lights stay bright then you aren't sending current to the starter motor.
Starter motor is commanded to crank engine by application of +12 Volts at the control terminal. If you have a volt meter or a simple light bulb, hook it from the control terminal to battery (-) (vehicle chassis) and try to start. You should see 12 V or light when the ignition switch is turned to the crank position. For those of us in a hurry, momentarily connect control terminal to the large solenoid terminal that is itself connected to battery +. Use a scrap bit of wire. Motor should crank.
If no crank look at starter motor harder, maybe bad solenoid...
So if motor cranks (or you see voltage/light) then you aren't getting control voltage. This comes from the starter relay which my FSM says is on the 'RH Shock Tower'. I don't know much about this or how to identify it. According to the FSM schematics, its contacts supply battery (+) to the starter motor control terminal. The coil is shown to be driven by the ignition switch on one side but the opposite side goes to a connector where I suspect it finds its way to the NSS. Usually the relay is marked in a way that indicates which terminals go to internal contacts and which go to coil. You'll have to figure out which is which on your own.
First, listen to relay while a helper attempts to start the vehicle. Hear a click? If so then it may be bad relay contacts. If not, suspect ignition switch or NSS or wiring in between.
Battery (+) should connect to the stud on the relay. You can verify this with the meter or light. This should always be present independent of the position of the ignition switch. When attempting the start the vehicle battery (+) should appear at either (both) of the other terminals connected to the relay internal contacts. It should not appear when not attempting to start. If you momentairly jump from stud to contact terminals with wiring connected the motor should crank.
If the motor does not crank, look at wiring to relay from battery (+) and from relay to starter motor solenoid control terminal.
Assuming the motor cranks then the problem is that the relay is not engaging, either due to a defective relay, bad wiring to the relay coil, failure of the ignition switch in 'start' position, or bad NSS.
If you can identify the lead to the NSS from relay coil, unplug this lead. Connect the exposed relay coil terminal to battery (-) (vehicle chassis) and try to start by using the ignition switch. If motor cranks then suspect NSS. If not, you may have the wrong terminal. Reconnect and try the same trick by disconnecting the lead to the other relay coil terminal and grounding the exposed coil terminal.
If motor still doesn't crank then the problem may be bad relay, or bad ignition switch, or wiring between ignition switch and relay.
If motor cranks then suspect NSS or wiring between NSS and relay.
Some warnings: Don't ground either of the wires to the coil terminals, only the terminals themselves with the lead wires disconnected. Be sure you have properly identified the coil and contact terminals on the relay. If in doubt about which contact is which, don't try... If any of the description above doesn't correspond to what you observe on your vehicle then we are talking about different elephants and nothing I've written here applies!
This is free info... take it for what is worth...
Now that you've read all the hard stuff try this trick: turn the ignition switch to the 'start' position and hold it there. Move the auto trans shift lever back and forth around park and around neutral position. Watch out, the engine may attempt to start in gear! You are sliding the NSS contacts around and they may intermittently contact and enable the start. My NSS seemed to be rotated a little too much where it attaches to the trans and didnt' make proper contact until the trans was nearly shifted out of neutral.
