Dakota 5.9 odd ground issue starter relay clicks but won't crank.

My 2000 R/T was fine until I shut it off at the shop a few days a go.
When I went to leave it just clicked.
I swapped the battery with a known good one and it still just clicked.

A couple of years ago, something similar happened, and it turned out to be the ground cable between the body and the back of the block/head.
I replaced that with a heavy gauge lug to lug battery cable between the compressor bracket (where one of the ground cables from the battery goes), to the lug on the firewall where that bad cable attaches.
That resolved the issue.

This time, I jumped that with jumper cables, but still just clicks.

Finally, I used the jumper cables to jump from the starter bolt directly to the negative battery terminal, and it started.
The starter was slow and the jumper cable got hot.

After I had it n the shop, I checked continuity between the body lug, the compressor bracket and the battery terminal.
All beeped, and then I Ohmed them out and all showed .01.

What's my next move here?
Am I missing a ground cable somewhere that could have failed?
Why did my heavy gauge jumper cable get hot between the starter case and the battery?