That 10 gauge green wire that Fred mentions is the ground. I experienced the same problem several times after driving through rain or over rough roads. The connection comes loose. I finally got tired of crawling under there to tighten it, so I moved it to where its easily accessible, somewhat out of the weather, and used a lock-washer and loctite. No problems since.
Be careful. Make sure hands and fingers are not in the line of travel.