What year?
How much fuel are you adding to get it from 1/4 to full? You may have a fuel leak. It could be the fuel level sending unit or a related wire, a somewhat common problem on GM vehicles. Easiest way to check is with a scan tool that shows the fuel level sensor voltage. You can also check at the connector with a voltmeter.
The odometer backlight, I believe, is an LED which is soldered to the board. You'll have to remove the cluster and take a look. Some places will repair them.
For the seatbelt light, try jumping the wires at the sensor under the seat, might have a bad sensor.