Trailer running lights out on one side

Running lights out on just one side of a trailer (not all of them) almost always points to a broken ground or ground wire that's corroded at the connector, since most trailer lighting shares a single ground wire per side. Check the 7-way connector pins for corrosion first — it's the highest-traffic failure point and the easiest fix. If that's clean, trace the ground wire along that side for a break or bad connection. Left side or right side, and are they all out or just some?