If you look at etrailer.com's Youtube videos they have a lot of videos on their various parts for the D40 trucks. It looks like in addition to the ground pin, the relays for the lights are the same as tail lights, left and right turn/brake, and ground are present on both and controlled by the three relays. The other three pins are 12V constant, trailer brakes, and "Auxiliary" that appears to sometimes be used for reversing lights. These trucks appear to be prewired for the installation of a trailer brake controller, not sure if 12V constant or Aux are set up for anything or not.
Based on these assumptions I would expect that all you need to do is replace the 4-pin pigtail from the connector on the rear driver's side with a 7-pin pigtail.
Based on these assumptions I would expect that all you need to do is replace the 4-pin pigtail from the connector on the rear driver's side with a 7-pin pigtail.
