Gas gauge not working properly and low fuel light is always on

This car has got a fair amount of aftermarket work done to it. But a lot of the wiring is not grounded right, but all I really want is my gas gauge to work all the other gauges like the mph, rpm, and temp needles all work. But the gas one does nothing. The low fuel light is always on even when the tank is full and the digital read for miles until empty just shows this " ---". I checked all the fuses that would have anything to do with it both under and inside the car. The entire trunk is dynomatted so trying to find the tank latch in the trunk is impossible so I cant look to see if the wire is grounded correctly then I was told the instrument cluster might of burnt out. All I am saying or asking is how do I check all this? My mechanic's hand is out of commission for awhile. So I got have learn. Can ya help me out?