Two wires from alternator to R/R OK and charge battery without lights. Third wire charges with lights on but gets hot enough to melt insulation. Suggestions?

 

How does the connector between the stator(alternator) and regulator look? Mine became corroded and started melting 1 of the wires once. I cut off the connector and hard wired it together after removing the melted section.

I would also make sure you don't have a short in the stator somewhere. I don't remember the measurements off hand, but you can download the service manual in the main 600f section and find it there.