I have 97 12v with auto transmission. The starter will engage whenever it wants to seems like around bumps. I have hook a light to the starter start wire and when it engages it doesn’t come on(comes on on 1st start) so I change the starter and main power cable to starter. Does the same thing. Checked neutral safety switch and that works correctly(found bare wire and fixed also). And still does the same thing. Any help or next steps would be greatly appreciated. Thank you. Update - I installed a push button about six inches from the starter on the small wire and it still does the same thing replaced wire so nothing is bare and showing. Could it possibly be the bolt for the adapter plate backing out? Have no camera to look in there and see if a bolt walked out

 

Take a look at the small wire that goes to the solenoid. It engages the the starter.

 

If it is engaging while driving over bumps, then it seems likely something is sending current to the small wire. Perhaps the ignition switch cylinder is rotating to the start position. Do you have a lot of keys on your keyring? I've heard of at least one case where the weight of keys caused the ignition switch cylinder to rotate and shut off the truck. The other would be to break out the wiring diagrams and start tracing wires - look for any place that trigger wire could get an unintended feed.

 

When I hear about starters making grinding noises when going over bumps, the first thing I think of is the starter bolts are loose.

I know you mentioned you replaced the starter, so it might not be the issue. I would still check it again, if it were me.