If you remove the plug from the sender unit, the gauge should drop to zero. As you've tried this, and it stayed at maximum, I'm thinking you have a shorted wire, or were unsuccessful in cleaning all the corrosion from the gauge cluster.
I have a '99 with the same problem, and mine goes to 0. CJtim states you should only use OEM's for these. I'm waiting a quote. Regardless, I'm not going to pay more than 40 for it.