Tail light question 1994 Astro van
 

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.