The Auxillary or Backup Light Circuit
The ground circuit is the first thing to check and repair if you have a wiring problem.
On a 7-pole RV plug/socket the center pole is used as an auxillary circuit and is usually used for backup lights.
Most tow vehicles that are factory prewired have this circuit, while many service centers do not include this circuit in a wiring package.
To test your tow vehicle socket, clip the ground clip of the test light to a suitable ground and probe the center terminal of the socket while a helper shifts the tow vehicle into reverse. Be sure that the tow vehicle parking and service brakes are applied to prevent any accidents when doing this test.
When the tow vehicle is shifted into reverse, the test light should light up if the plug is wired for backup lights.
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