Do yourself and you car a favor and bypass that old amp guage. It's a fire hazard waiting for a place to happen.
A quick and easy test for now, take the left connector wire off the amp guage and move it over and connect it on the right terminal with the existing wire - ie, both on the right hand stud. This will by-pass the amp guage and take that potential problem out of the equation. The other thing to check is your neutral safety switch - is it connected and working? And as the other guys mentioned, the fusible link. Good luck.