2004 Toyota Celica Gas Gauge Not Working

I have a 2004 celica Gt. About a year ago I had an alarm installed at a shop. Before I had the alarm installed, the gas gauge worked perfectly fine. After the alarm was installed, my gas gauge no longer showed how much gas I had on the vehicle. Everything else on the cluster works fine. Sometimes when I would turn the car on, the gas gauge would work. If I hit a bump or something though, the gas gauge would go away again. About 9 months ago though, the gas gauge completely stopped working and I have not seen it work since then. Any thoughts about what it could be? Could it be that the guy fried something while installing the alarm? Any thoughts about how to go about fixing it? (My mechanic replaced the fuel sending unit on the gas tank and still no gauge. He also checked for continuity from the gas tank to the wirings under the dash board and there was continuity all of the way through). Any other ideas of what it could be?