The connection is to the green/yellow wire in the A-Pillar, sourced through the dome light fuse of the Main, underdash harness. As John already mentioned in your other thread pertaining to this light, the original wire was wrapped with a black, cloth-like wrapping. It would also be taped aprox. every 12 inches apart to the tailight harness till it exits the trunk above the drivers wheel well.
As with so many details you are seeking answers to, this information is clearly spelled out in the Osborn Electrical Assembly Manual.
I held off buying a full set of these manuals too for a while but it might be time to consider getting a full set. (Laurel Mountain Must. sells a full set of 6)Clearly, you want to get it correct. After buying a set and reviewing them, bring the questions you might still have here and we can help. Many of us already travelled this road before. As of lately, much of the help you have been getting is sourced directly to those manuals. If nothing else, you would have images and page numbers we could discuss openly a whole lot easier than by hit/miss trying to type out these answers.
Richard