How did u step down the voltage? LEDs are normally 1.5-3.0V dc, and the longest pin is positive, unless yours are different. Maybe you bought them pre-made in T5 casings? (or did you retrofit them into the cluster's old T5 sockets?). I'd imagine when replacing bulbs in the cluster, best thing to do is just solder the LEDs direct to the PCB, they won't need to be changed again
Its not possible to do the illumination ones whilst one's in there as the dimmer screws up the LED voltage feed- end result is they will never dim like the bulbs do.