With the 4, 6, or 7 pin trailer wiring systems the ground for the lights is the frame and the white ground wire in the connector.
The "common ground" connection on the plug is only hard wired to the battery charging line, breakaway switch and the trailer brakes and then bonded to the trailer frame.
The running, turn and stop light grounds are through the frame.
However, that wire connection between the frame and the the ground pin on your connector is your weak link. If that connection is corroded you won't get a return path for the lights without being hitched.
See attached wiring diagrams.
Lou and Laura with Bella - German Short Hair Pointer
2008 GMC Sierra 2500HD Crewcab SB Allison Duramax
2010 Flagstaff 8526RLWS - Superglide 3300
HAM CALLSIGN - KC3FFW