You need to hook up the rear two function wires (off the 6-pin harness) to power the rear ditchlights. If you plugged the LM into the 9 pin harness on the board, but didn't do anything with the 6-pin on the LM, then you only have 4 of the 6 functions hooked up. The four functions will control the headlight, rear light, and front ditchlights. The additional two are required for the rear ditchlights.
You will need to either solder these to the board (not sure if it has pads for that....need a picture), or solder the wires directly to the rear ditchlight wires WITH a resistor. Otherwise you will blow the bulbs immediately. I'm assuming these are still Athearn's bulbs and not LEDs??
-Kevin