Here are the steps to replace a tail light bulb on a Challenger: (Part #3057) Remove the large wing nut from the side interior trunk panel behind the taillight. Pull back the panel, remove the large wingnut behind the panel. At the top of the taillight, the trim piece has 2 tabs that have to be moved up. Now pull the taillight straight out. You can disconnect the plug if you want, push/slide the red retaining tab and then wiggle the plug off. Turn the bulb plug counter-clockwise until it pops loose. The bulb pulls straight out. Replace. Reverse process to reinstall.
SouthernCarParts sells a plug and play harness for $149.95. Where did you get your kit and how much did it cost? https://www.southerncarparts.com/dodge-challenger-sequential-tail-lights-p-2390.html
I checked out this website. It looks like SouthernCarParts isa selling the same kit for $149.95. Here is the installation process. https://irp.cdn-website.com/14c4291a/files/uploaded/STS-4CHSS.pdf
^^^ Technically, THIS is the "earlier" Challengers I wanted but never did add the extra to my early '70 Challenger but never did