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This is the primary cause of trailer-light problems. Symptoms include flickering, fast-blinking, dimness, intermittent operation (often when a trailer ball is used for a ground), and complete failure.

PITTSBURGH, April 17, 2025 /PRNewswire/ -- "I'm a mechanic and was frustrated with the inability to check trailer light circuits without a semi-truck connection," said an inventor, from Spartanburg, S ...