[Quicksilver]

You need to test basic systems. Get a long lead with clips to hook on under the car and use a multimeter on it. Take notes and where the issue is will make itself apparent.
- Connect to the starter switch lead on the solenoid and make sure that you are getting power when you try to crank. There will be a voltage drop but it should show some level of consistency that will be a clue.
- Connect to the battery and see if you get a consistent voltage drop when the starter engages vs when it doesn't crank. An automotive starter is an overloaded device. It draws a huge quantity of current and can't be run for long before the starter, cables, and battery will melt down.
- Connect to the solenoid's battery connection and check for voltage drop cranking vs non cranking.
- Connect to the starter's connection to the solenoid and check voltage cranking vs non cranking.
Because of this you can easily see if the starter is actually being engaged because it will cause a noticeable drop of voltage.

With those four things you can troubleshoot most all of this. Assuming that the battery voltage doesn't show an obvious battery problem those four voltages are all you need.
You should see a reduced voltage drop when the car doesn't crank because the needed power isn't getting to the starter and we need to follow the chain of the other tests to see where it is going wrong.
- If the voltage drop at the solenoid's battery connection is noticeably higher then at the battery then you have a battery cabling/connection issue. Otherwise look farther down the chain.
- If you don't have voltage or only very reduced voltage at the starter switch's connection to the solenoid then you have a switch or wiring issue. (Compare voltage at the ignition switch to determine which.)
- If you have a reduction in voltage from the solenoid's battery connection to its connection to the starter here is a problem with the solenoid contactors. (Thank you Porsche for making a solenoid that can't easily be rebuilt.)
- If you have full voltage to the starter but it doesn't crank it means the starter isn't trying to crank and you have a brush, commutator, or winding issue.
or the starter isn't drawing current because of a brush/commutator or winding issue.