There's grounds everywhere. Black cable would be ground. Metal lands where you screw the board to the case would be ground. Case itself would be ground.

I wonder about the dfi's readings though. I'm thinking that it might be reading it deeper in the circuit somewhere, and it's already taken a hit in terms of voltage drop due to resistance in the circuit leading up to where its measured. So, that said, I wonder if that's actually what many components are seeing, or if the poor conduct is due only for the measuring system.