my 69 chevelle cranks only some times?

check power to starter and solenoid..replace the solenoid or turn the carbon piece inside around and stretch out the spring a little bit...or just put in a mini starter for $75.00 good choice... another thing to do is run a ford solenoid to the stock starter/solenoid...this will stop the power wire from being live all the time

If the starter is fine and spins motor over but it will not crank confirm what is missing fuel or spark, if it has both maybe its out of time. I assume it does not need a full tune up or ignition parts. Starter not working, check grounds/connections, may be on its way out.

Starter doesn't turn over. then once in 20 try's the car starts

May be the ignition switch or its wiring to & from or loose connection, lost ground from block to frame or other frame grounds. Starter/solenoid, When it does it jump the power across solenoid to stater with a screw driver (sparks so uno) can be done by using a manual starter button but it has to be installed first. If starter works its the solenoid on it.

If you jump start the car it starts automaticaly

New starter, battery test good. Light don't dim if you try to start the car

Radio plays when turned on, the shuts off when trying to start the car

Switch needs turned on to jump the solenoid if you want it to crank up.

switch? ignition switch.

Ground issues or shorts across ground wire sending power to frame. Put a multimeter on volts dc black probe on neg- and red probe on frame and body see if you have voltage. May not and radio is hot-wired to hot+ power from battery before the fuse panel and is not on a fuse after power is received by the fuse block and the power is draining so far from battery to cause the radio not to work while cranking, so that tells me there is power loss/short when ignition switch is on or weak battery. The short itself could be any component or wire, could be the starter too. There is some youtube video's showing how to narrow down shorts by using the fuse panel to check. Weak ground wires/connections can do this too. Battery terminals & terminal wires, weak battery with bad cells.

Does have a old school Neutral Safety Switch / Range Sensor too it could be going bad, sometimes you can tell if is by trying to crank it in gear or better yet since its starter sometimes does not work try going threw the gears first then try say like in neutral or back in park or scotch tires, foot on brake, try it in gear to crank it, if it does or tries its bad or going bad but may or may not be the actual cause of this issue but still could be too.

http://troubleshootmyvehicle.com/gm/4.3L-5.0L-5.7L/how-to-test-the-starter-motor-1

If you can test the starter out as the above URL points out, may help out. The starter solenoid is a switch that when you turn the key send power to it that completes the circuit for solenoid which closes it and allows the power from battery to reach the starter motor. If turning the switch does not send power to the solenoid then you problem is narrowed down to point to the neutral safety switch or ignition switch or those wire harnesses. If there is power to the solenoid (small post) and jumping the 2 bigger post with a screw driver sends power to starter and it works(starter) its your solenoid thats bad. Also jumping the 2 large post on solenoid does not engage the starter then the starter is bad, unless weak battery power (either weak battery or power lost(bad grounds ect). Use voltmeter black probe on frame (unless parasitic battery drain detected as above test few post up showed how to do) or negative battery terminal and to where the power wire from battery connects to solenoid if less then 10-12 volts (should be 12+v on a new terminal wire), power is lost threw terminal wire, battery terminal or weak battery or huge parasitic battery drain. If low voltage to starter solenoid a jump off from another car should crank it right up.

Did you replace the terminal wires? If not do so.

I think you need to disconnect that radio. Hook it up correctly. Change those terminal wires (burned up or corroded or frayed or loose) and the battery terminals too they are metal and after a while they loose the ability to carry juice (they get hot & corrode).

I was typing then saw after 3 post up you had posted it jump starts up. Your not getting enough power to starter, you may be loosing it somewhere other than terminal wires(may be loose/dirty at stater) & terminals but if they are burned or old or corroded I would try that first, they can be corroded down under the insulation. If that does not work then locate the short (which should dim the light just as the radio stops but hey No telling how where or way its wired up).