The third brake light on my '93 sucked when I got it. Some of the LEDs were trying harder than the others, but they were all pretty dim. I too took it apart expecting a smoking gun (moisture, corrosion, something burned up inside) and didn't find anything obvious. LEDs have a life span like anything else, and I figured mine had just worn out over twenty-odd years and 230-odd thousand miles of urban driving. I had a good one in my parts car, so I didn't spend too much time troubleshooting the bad one before swapping it out for one I knew worked.
I doubt the issue is one string drawing excessive current. I think you'd see evidence of heat or blown fuses if that was the case. I'd verify that the voltage across each string is the same as the supply voltage (to rule out high resistance in a joint or trace or something), maybe test an individual LED on 3v to see if it lights up properly, and if both of those come up negative, order a bag of red 5mm LEDs and get ready to do a whole lot of soldering.
If that doesn't sound like fun, there are aftermarket ones on Fleabay. They've got red ones, they've got black-tinted ones if you like painting your lights so they work less, and they've got clear ones because Altezza, yo. I didn't check to see if Nissan still sells them, but yeah, I wouldn't be surprised.