The tail light you got with the fender should have a clear illumination lens on the upper side. This is to illuminate the license plate. If there are three holes above the tail light, this is for the license plate bracket. I reused both my tail light and my license plate bracket. If you haven't noticed yet, the tail light is the "Center of the Universe" for H-D rear wiring. Everything plugs into it. If your existing harness doesn't reach the tail light, there are replacements for reasonable $$. I reused my Street Bob harness and reglued the foam strip.
You are going to start realising just how many parts H-D uses over and over again across models and lines. The tail light for example is the same for Electra-Glides, Softails, Dynas and Sportsters. Just pick what you like from each model and create what you have always thought of as the perfect Harley