Why isn't my Fuel gauge on Mazda truck working?

Assuming it is a 86-93 model bseries I will for answering this.If the gauge does not register it is more than likely the fuel sending unit in the fuel tank is BAD.To correct this requires dropping the fuel tank and replacing the fuel sending unit.You can try and ground the power wire from the harness at the tank(very difficult to do for clearance purposes) and if the you ground the power wire and it makes the gauge move to the full position the fuel sending unit is defective.I would just buy a new part and drop the tank being the truck is probably over 15 to 20 yrs old and needs a new fuel sender.I seriously doubt the gauge is bad(in most cases the sending unit is worn out in the fuel tank).Ok