If you don't get the "self light" test when the ignition is first turned on, then the bulb may be shot.
The reserve light is controlled by simple switch contacts near the bottom. A search here on the forum should give you more information on how to clean them.
My recommendation is to not rely on the reserve light, even when you have it in normal operation. I ran out of fuel one time on a car that reliably lit the reserve light - except for one time.
Ken300D