Tail light : 3 bulbs found : Just wondering what the purpose of the middle light bulb which has a pigtail and a small glass bulb. Description/diagram of the Astro's tail light is not in shown in the manual and I could not illuminate the middle bulb with any switches. Top bulb runs parking, hazard, and brake lights and the third /bottom bulb runs reverse light with a clear lens. Thank you.

 

I know this is an old thread, but maybe it will help someone. I believe the third lamp you are referring to is the side marker lamp. It should illuminate when the lights are in the park and headlamp on position.