My 1999 Prowler won't start, and nothing electrical works. The battery voltage is 12.4V

How do you know the battery is fine? Sure sounds like you have a weak battery- the symptoms are all the electrical bits work- except starting the car. All your electrical systems don't need much current, but your starter needs a lot. If your battery isn't fresh then your car won't start, but everything else will appear to work. You can test the battery by looking at voltage across the terminals when you try to start the car, if it's less than 12 volts your battery needs to be replaced. If your battery is more than 5 years old it should be replaced anyway. Good luck, let us know how it goes.

Even though all your other electrical bits are working it could be that your battery doesn't have enough juice to crank the engine. It doesn't take much power to run the radio, lights, etc, but cranking the engine requires the battery to have a very strong charge. If your battery is more than 5 years old, that's very likely the issue. I'd start there. Also, be sure to check the battery terminals and ground for corrosion and clean if necessary. If replacing the battery doesn't solve the problem it could be the starter solenoid or a loose electrical connection somewhere.

You would need to use a meter or test light to find out what exactly is dead. It could be a short in the fuse box, or a bad connection in the harness, or at the battery or starter.
You may need a repair book or shop manual to get the wiring diagrams.
If nothing on the truck works, you would start at the battery cables and work back to the key switch.

At www.reliable-store.com get complete guide on whether to repair or not,
service it, and how to repair etc, along with wiring diagrams, electrical diagrams, torque specs, error codes, everything.

Check the battery's state of charge-might be too low. Check battery post connections. Clean and retighten them, and check battery ground, and connection of cable to starter. Sounds like starter is not getting full battery voltage to work.

How about corroded battery cables (internal)

Fusible Link wiring corroded & open circuit,does
everything electrical work except the starting circuit?

Turn key & have power to starter solenoid ?
Neutral Switch !
Ignition Switch !

Tough to guess without wiring diagrams,
to see where to start testing

Can't fix what you don't know you have

If the car does not have a security system, I would check for voltage at the starter from the battery and from the key switch.
The power from the key switch goes thru the neutral safety switch before getting to the starter.

if starter is not engaging or clicks then have battery load tested. if battery checks good then test the voltage draw on the starter,may be dragging when it gets hot.

Battery , cables , connection's could be bad. High resistance in a cable , connection , Ground connection Corrosion ... Voltage drop test the electrical circuits .
Voltage Drop Test
Voltage Drop Testing the Starter

Go under the dash and bypass the clutch pedal safety switch and see if it starts.