Whether there were 5 or some other # in the factory 3rd light, I don't know, but yes it should have enough to illuminate nearer to the whole width of it.
I'd pull it off and see if water got inside. Might be sealed and need heated up and some prying to get it open to DIY repair, or just give up on that one and replace it. I'd try the repair first because $100+ seems like a lot for what it is, probably just some inexpensive LEDs and resistors in series that you could solder up to fix then seal it back up with RTV sealant or epoxy.
The ideal replacement LEDs (if needed, might just have some PCB trace corrosion and the LEDs are fine) color would be red. White would work but a lower % of total light will make it through the red lens.