Celis tail dimmer than the other

[nine9six]

I have retrofitted the Celis tail light on my 00 e39 and just recently, I noticed 1 side is much dimmer than the other. The dim portion I am talking about is the 4 strips that are lit by LED. I tried to switch the "ballast" from one side to the other and the same side is still dim. What can it be? Can all 4 LED be going bad at the same time on one side?

[mattmartindrift]

one light might be newer than the other -- an accident might have replaced one before the other.