Starting

If the starter will not disengage, you have continuous power to the smaller wire on the solenoid. Using a test light, confirm that.

https://www.youtube.com/watch?v=Hlna_kUt7As

If you confirm that there is always power there, even with the key in the run position, check for shorts to power. If there is not a short/or power all the time, I suggest removing the starter and bench testing it to see if the Bendix returns to the off position when power is released. You may have a broken return spring allowing the Bendix to stay engaged with the flywheel. Most parts stores will test it for free at the store.