Alternator Dance help needed

[mercoupe50]

I have a problem with the charging system in that it has wide swings pos/neg as seen on the amp gauge. The lights go bright and dim suggesting that the voltage is actually fluctuating. The alternator belt is tight and not slipping. I have changed the alternator and the regulator and still have the problem.
The problem shows up at fast idle. Turning on the headlights and everything else to make a load makes it dance less.
It does not happen when the battery is charging when the battery is low. As soon as the battery comes up the needle dances again.
I checked the wire going from the battery to the frame. I found that ok.
There is a heavy gauge wire attached to the frame outboard of the starter that runs to the block. It is ok as well.
In the dash wire loom there is a ground wire (brown) that appears to be about 12 ga. or so. Where does that wire pick up frame ground? I really can't find anyplace where the brown wires pick up the frame ground although I know that there must be a place