Typical LED taillight wiring:
RED: "Bright" side of lights (brake/turn signals).
BLACK: "Dim" side of lights (Parking/tail lights).
You are correct, the old ones grounded through the mounting screws. In my case, when I swapped to LED's, I used the grounding points previously used by the 8-way flashers (since I had the inner sheetmetal off, and the 8-way bulbs were removed and had relatively easy access to it.)