There are 2 standard trailer plugs; 7-way RV and 4 pin flat.
7-way RV plugs are designed for trailers with electric brakes. These systems will usually have 5-7 wires in its harness
4 pin flat plugs are designed for a trailer that has basic lighting requirements.
The common wires in a trailer lighting system are:
- Running lights (taillights)
- Stop/turn RH
- Stop/turn LH
- Ground wire
Systems that have electric brakes have the following additional wires:
- Electric brake wire
- 12V charge line
Common problems in electrical systems are, faulty connections, grounding issues, worn or pinched wires, burnt out bulbs, faulty lights and damaged or worn out plugs.