gas tank ground wire?

Use a set of jumper cables for the "test". Clip them on the tank and to a good ground. THEN, if it still doesn't work, remove the wire from the sender and stick a small Phillips screwdriver into the plug and ground it. If the fuel guage reads "full" you have a ground issue, a bad sender, or a sunken float. If it doesn't, your problem is in the dash wiring or the line back to it.