The close-up images aren't as clear as you would expect (or hope for) given all the painting and weathering and patching that's been done over time. But, I strongly suspect there's a thin plexi strip insert at the top of each segment along with a wider bar below with a vertical corrugated texture. These are the areas that light up.
The thinner strip on top has been taped off and painted over so the area that actually lights up is even thinner than the plexi insert itself. They also used tape and/or paint to divide the lower strip.