Turn the power off. Remove the flourescent assembly (3 scres and a connector). Measure the ohms between the two wires coming out of the ballast. If it's good, it will read approx. 25 ohms. If it is zero or infinity, then it needs to be replaced.
If it reads okay, then turn the cab on and check the AC power at the connector from the cab. It should read approx. 120vac. If it's lower than 75vac, it probably won't light the bulb.
If you have a good ballast, good power, and a good bulb, then replace the FS-2 starter. Reinstall & reconnect the fixture and you should be good to go...