They're usually a stud mounted on the body with several brown wires going to them. There's a couple under the hood, and some in the trunk, and some under the dash. Sometimes loosening them, wiggling the wires and re-tightening them works, or you could completly take everything off, scrape them off, clean them up, coat them with grease, and put them back.

Anytime you loosen a ground connection disconnect the battery first.