Re: tail lights
My guess would be that it isn't an original RS car, and when they needed the wires to hook up the backups under the bumper, they just chopped and tossed the socket. That's the only reason I can think. It's easy enough to get a replacement tailight socket (three wire if you want stop/turn and parking, two wire if you only want parking) and splice it in. That's what I did on my 68 when I hooked the RS backups in. It might overtax the old existing wiring, but it didn't on mine.
The best way to do it correctly would be to get a 67 RS convertible rear body and lighting harness like the FactoryFit that American Autowire sells. It's just over $100 and it would get rid of any broken wires or splices in what you have. It could be a bit of a snowball project, because it runs from under the dash all the way back, so you have to remove the driver side kickpanel, door sill plates, wire gutter covers, top and bottom of the rear seat, and the rear arm rest panel on the driver side. But, you'd get it taken care of and not have to worry about splicing and shorts, etc. You'll probably spend $25 on parts and 3 hours or so to splice some sockets in, vs. $100 and 8 hours or so to do it "right."
69 'vert project big block/TKO 600 RR
68 'vert driver RS clone -- gone!