not starting

I have a 99 cavalier with a serious and confusing problem that has me and others confused.
I turn the ignition over and nothing!
the starter has been replace and worked for a while then the same thing.
Next I replaced the neutral safety switch which seemed to work for a while then the same thing again.
I noticed that when I disconnect the battery and reconnect the car will start but when shut off nothing!
Tried the battery thing again and it worked.
There is power to the starter 12v.
Maybe relay.....ignition moduel?
Is there a way to test them before replacing$$$
Does anybody have an idea????pease help me:banghead:

I'm not sure I understand exactly what you are describing. When you try to start does the engine turn over. Or does nothing happen. If nothing happens when you turn the key to start. Check the ignition switch you can use a voltmeter and check for voltage from the ignition switch when you turn the key on.
Look on the steering column just below the two bolts that connect to the dash. A rod from the key tumbler pushes it to start.
Auto Zone or Advance check modules free if you take them in. The camshaft position sensor and crankshaft position sensor can be tested with an ohm meter. Disconnected with the ohm meter across the two contacts it should go from zero to one when cranking.