No Crank?

At this point the most likely suspect is the neutral safety switch. First try cranking the engine with the transmission in "neutral". If that doesn't work, first double-check if the back-up lights are working. That switch is in the same assembly. If both systems are not working, there's a good chance the plug fell off the neutral safety switch. It's on the driver's side, rear of the transmission, just above the pan, and has three wires in the plug.

What are the symptoms or observations? Do you hear a single, rather light click from the starter relay? How about a single loud clunk from the starter each time the ignition switch is turned to "crank"?

The starter system can be broken down into four parts, each with a test point at the starter relay's socket. You can use a voltmeter, but for this type of problem, a test light can give more accurate results. Start with the test light's clip lead grounded to the battery's negative cable or to the engine or body, remove the starter relay from its plug, then probe the four wires in that plug. One will have 12 volts all the time. A second one will have 12 volts when a helper turns the ignition switch to "crank".

If you find those two terminals are okay, move the test light's clip lead to the battery's positive cable, then probe the two remaining terminals in the plug. The test light should light full brightness in both of them. If one terminal gives the wrong result, tell me which color that wire is.

If all four of those tests come out okay, the last suspect is a corroded negative battery cable. There's actually two of them, and either one being corroded off can cause this. The larger one goes to the engine. The smaller one goes to the body sheet metal. Move the test light's ground lead back to the battery's negative cable, then probe the engine block while a helper is holding the ignition switch in "crank". If the test light has even a slight glow to the filament, that cable has wire strands corroded away, often hidden under the insulation.

Next, probe a rust and paint-free bolt head or other point on the body. We need current to want to flow, so turn on something, even though it may not work. The head lights are the best thing as they'll want to draw at least ten amps. That is plenty to make a defective smaller battery negative wire show up. The heater fan is another good one to turn on. When there's a problem with that smaller negative wire, there's usually other symptoms leading up to the failure. All the exterior and interior lights, and all the other circuits need that smaller wire.

The only other thing that can do this is the larger positive battery cable. You said you found 12.5 volts at the starter. Is that the large battery cable or the smaller wire? There should be full battery voltage on the larger one, but all it takes for that to show up is one tiny strand of wire that isn't corroded away yet. You'll never get 150 amps through that to run the starter. The clue here is you'll still get the loud single clunk from the starter solenoid, but the voltage on the fat cable will drop to almost 0.0 volts. You have to measure that voltage while the starter is engaged. If that 12.5 volts drops real low when your helper turns the ignition switch to "crank", that positive cable is suspect.

For all of these tests, there are a few more things we can do before spending money on parts. Let me know what you find with these observations and voltage readings.