How to control this 72-LED light bar PCB
My probing is beginning to bear fruit. Here is what I have discovered so far:


My multimeter's leads are too blunt to make good contact; jumpers from my Arduino kit work better.
The LEDs seem to be divided into sets; within each set the LEDs are wired in series.
Pins 1 through 7 on the ribbon cable are electrically connected inside the PCB and form the positive power supply (approx. +12V)
All the red LEDs are a set (I can get up to 3 to light up at a time with 5V from my Arduino)
Every ninth green LED is part of the same set.
Pin 8 is the ground for the red LEDs.
Even with 12V, only 7 out of the 8 red LEDs will light (I have to touch one of my power leads to the second-to-last LED rather than the ribbon cable). With the green LEDs, only 6 out of each set of 8 will light with any reasonable brightness. I assume that with higher voltages I could get them all to light at once.


The two light bars are slightly different (the ground pin details above refer to the one labelled "R/R") but they are very similar otherwise. The only major difference seems to be that the "L/F" one has two sets of red LEDs and the "R/R" one has only one. Thanks for your suggestions; I think the mystery is solved.