96 grand prix won't crank.

This sounds like the starter solenoid isn't getting enough power. Jumping the solenoid is you simply bypassing it. Go back to basics. I may suggest things here you have already done but it helps to treat this as a simple check list.
- Check the fuse, sometimes the fix is that easy.
- Check the relay for the starter motor. It should click when the key is turned sending power to the solenoid.
- Check the voltage supply at the small wires at the solenoid. Using a multi meter set to DC Volts you should get around 12 volts when you turn the key. You might need a helper to do this.

have you checked power and signal at the starter motor, the large wire @ the starter will have battery voltage at all times and the smaller "signal wire" will have power only when the ignition cylinder is being cranked, if both ok then you will have a stuck/seized solenoid on the starter . lightly tapping the starter motor while "cranking " will sometimes allow the vehicle to start.

Better check with a test light for power to the small wire that goes to the starter from the key switch. If no power need to check for a neutral safety switch or fuse. Or the starter you got is bad. Alternator will not cause not to crank over

I would check the starter for voltage first - there is a starter relay that is grounded by the PCM. If the ASD relay is faulty the pcm won't have power. You should have battery power to the starter and power on the solenoid when you turn the key to start.

Check the neutral safety switch on the transmission. It should have three wires on the connector (if I remember correctly). There should be power on the Brown with yellow wire with the key in the crank position. If there is ground that wire and see if the van starts. If it does replace the switch. If it does not check the starter relay. Let me know if you have any questions and I'll do my best to answer them. Thank you for using FixYa.com!

I’m happy to assist further over the phone at https://www.6ya.com/expert/jeremy_d728a59f986299fa

you never mentioned if you have power from the switch, check for power in and out of switch with volt ohm meter, then trace wiring to starter

When you turn the ignition key to start your car, voltage from the battery flows through the ignition switch to the Park/Neutral safety switch and/or brake pedal or clutch pedal safety switch (you have to push the pedal down before the circuit will complete) to the starter relay or solenoid. When the relay or solenoid is energized by voltage from the ignition switch circuit, it closes a contact that routes more power from the battery directly to the starter to crank the engine. The starter motor spins, pushes the starter drive gear to engage the flywheel and cranks the engine.

Low battery (Check battery voltage, recharge if low, or jump start with another vehicle or battery charger).
Loose or corroded battery cables (Inspect, clean and tighten BOTH ends of BOTH battery cables).
Bad starter relay wiring connections or ground connection (Inspect, clean, tighten wiring connections).
Bad starter relay/solenoid (Check for voltage at relay, if relay has voltage but there is no "click" when key is turned to start, replace relay).
Bad starter (Jump battery voltage direct to starter to see if it spins, or remove starter and have it bench tested at auto parts store).
Damaged starter drive or teeth on flywheel (Remove starter and inspect drive gear and flywheel teeth, replace damaged parts if necessary).
Bad ignition switch (Check to see if voltage reaches starter relay/solenoid when turn to start. If not, check for open P/N switch and brake or clutch pedal switch. Replace ignition switch if defective).
Open P/N safety switch, or open Brake Pedal Safety Switch (automatic transmission) or open Clutch Pedal Switch (manual transmission). Bypass switch with jumper wire to see if engine cranks, or use test light or voltmeter to check for voltage passing through switch when ignition is turned to start.
Engine seized due to bearing failure or internal damage (Use socket and long handle to see if engine can be turned by hand, if not engine is locked up).
Engine hydrolocked due to coolant leak from leaky head gasket (Use socket and wrench to see if engine rotates, remove spark plugs and see if coolant comes out or engine can not be cranked with plugs out).

Bad solenoid i would guess. Not making a connection there. Any basic wiring and ignition switch problems should be alleviated by jumping the starter with a screwdriver. I would check the solenoid and starter brushes

this may be a ignition switch see if the little wire going to the solenoid gets power when ignition switch is tried

YOU HAVE TO DETERMINE IF THERE POWER TO THE STARTER SOLINOID. IF NOT A BROKEN POSITIVE FEED WIRE OR SEIZED. HAVE YOU EVER REPLACED THE STARTER