Under the outside LED boards on the right side, where the cables would exit the bar, is where the control board would be located. The main power cables and the serial control cable (which is just an ordinary Cat5 network cable) all attach to that. There are smaller wiring harnesses that connect the LED boards to it.
Do you have all the internal wiring? If so, all you should need are the main power cables, a network cable, the control board, and the serial interface module. I am very close to parting-out one of the Legend bars I have due to marginal, flaky LED boards. The control board appears to be fine, it got through the lightbar test sequence. I have a new serial interface module as well, I can sell most, if not all, of what you need to complete your bar.