Regal won't start by itself

94 Buick Regal. 3800. Couple days ago worked fine, 15 mins after last drive won't start, just clicking. Replaced battery. Replaced starter/solenoid, negative bat cable. Won't start on own with not even any clicking. Will start no prob with boost(only with another car, charger boost clicks and dash lights go nuts) and continue to run. But won't restart on own. And no clicking now.
Have had charger out and on . Cleaned + connection.
Both batteries have a reading of 12.3-12.5 when tested by themselves but only read 9.6 when connected. Test was done at screw heads. When running test shows 13.9-14.2 v.
Also when trying to start alone not even any idiot lights in dash come on.
Again volt tests were done on head of screw and starts no prob with boost and runs good.
Replaced pos cable and doesn' appear to be it. But without the smaller pos cable connected to the fuse box beside the battery that has the boosting post, the battery tests 12.5 v. and the pos cable to the fuse box on the pass. side also. Hook up the post cable and reads 9.5 v.
Likely a ground issue? And likely the fuse box beside the battery as it only happens when that's connected?
Unhooked that box from fender wall and don't see any ground and all wires appear to head toward firewall.
With there not being any dash lights or clicking(anymore) when trying to start yet start and run with a boost, does that point to particular fuses that may be grounded.
Any thoughts?

Based on what you've said I'm convinced the positive cable is bad or really corroded beyond repair w/o replacement. Your voltage drop at the screw heads supports this as well and the flickering dash lights also suggest an intermittent connection. When you jump your car using the jump post you are bypassing the screw to battery connection in a sense (but not totally) and your power is coming from a different source (as in not your battery)