I have custom LED tail lights (LED fixture, not just bulbs) for my 04 Civic. They've been working fine for the last few months. Just recently, the right tail light BARELY shows any light when I hit the brakes, but the left one is fine. If I turn on the lights, it lights up properly, but does not get brighter when I hit the brakes (but the left is fine).

I checked the connection and reseated it, but nothing changed. I'm tempted to break out the multimeter to check voltages between sides, but not sure what else it could be...?

 

That would be your best bet to compare the voltage to see if it is the actual current or not.

 

sounds like a bad ground