You'll need a variable resistor tool to check the gauge. Unplug the connector in the rear at the tank and hook the tool up with one lead connected to the purple wire and the other to a known good ground (usually there is a ground wire by the tank). Then, with the key on, dial the resistance from 0-90 ohms and see if the gauge moves. At 0 ohms it should be at empty, at 45 ohms it should read 1/2 tank, and at 90 it should read full. If it does not, then it is most likely a cluster problem. If it reads normally, then the issue is the sending unit in the tank. As far as the light, you need to make sure the gauge is working first, then deal with the light. It may be a bulb out, or it may read once the gauge does. If the gauge does not read properly during testing and you want to make sure the wiring is ok, you will need to disconnect it at the other end and then ohm check it with a DVOM from the purple wire at the tank to the other end.