Question about lights in speedometer

[COBALT]

Sorry...not the speedometer. The gas gage.

I finally got what I think are ALL of the lights finally working in my instrument cluster. So this may seem silly to a lot of you, but it's actually cool to finally get about 90% of the electrical systems working - especially under the dash after owning the truck for 7 years without a radio or decent heater controls. I've also been driving around with half of the bulbs burnt out in the instrument cluster.

Anyway my question: there's two lights in the gas gage. The one on the left says "brake" and comes on when I turn the key, but goes off immediately after the truck starts and I let the key go. The other....I don't know. It doesn't light up - ever.

What's the light on the right, and when should it come on? When should these lights come on and stay on? I don't have an owner's manual so I'm obviously trying to figure out what these lights are supposed to mean...