In the mid-1960s Chevy introduced the use of fiber optics in cars to monitor taillight status. The system was called the "Vigilite" and it was super cool high technology that Airstream decided to incorporate starting in the mid-1970s as turn signal indicators.
There's a small indicator on the side of your trailer that can be viewed in your rearview mirror. It was originally covered with a white plastic shell that degrades and cracks in the sun. This formed aluminum cover will replace your worn out cover.
Measures approximately 1-7/8" x 1-3/4" x 5/8".