Are all three cables intact? If not, it will wobble until you replace the broken one(s). If yours is a '67 or newer mirror, that's not an easy proposition. I don't know where you would get spare parts for one or if you can even remove them without breaking something. When the mirror in out of the car, there is no tension on the cables so they'll appear to be loose but once it's installed, they'll tighten up and hold the mirror firm. If it's just the glass bracket that is loose, tighten the screw under the base. If the glass has come loose from it's backing, you can probably reglue it if it will pry out without breaking. New mirror glass is cheap from any glass shop.