Starter clicks using the key, engine starts by jumpering the relay

I've noticed when the second small ground wire from my battery to the core support on my 72 is disconnected, and on my 83 pickup, I get strange behavior like that involves the turn signals.

I think you said the big negative to the block is fine and clean. What about that smaller one that usually grounds near the radiator?

I had a dead short caused by my clutch linkage which melted that wire at one point and I played hell finding it because it's not very long and isn't in a place my eyes land often when looking around under the hood..

Just a thought in case it helps.