I traced the wires back and found the fuse block was heavily corroded.
The ground was not adequate.
I don't understand DC that well but I suspect that could cause the voltage drop.
New fuse block works wonders.
The other part I had to learn was the stereo has a ground (black), power (red), and 'always on' power for memory (yellow). I didn't realize you need to hook up both the red and the yellow. The red is operated by the breaker and the yellow is not (of course).
The Sony instructions was not that clear, but a coupld minutes on the phone with Craig was terrific.