You don't need the factory harness if all your looking for is running lights, infact you really don't need it at all. I install hitches on these rigs quite often at my shop and there is aftermarket pieces that are available. The only thing that I have seen going OEM offers is that it will disable the PDC when your in reverse, lock out the ride height control, and on some will monitor the lighting of the trailer, that may have been the e70's tho.

I have a 7 way with charge line on the back of my e53 with brakes being controlled from a p3 control mounted in the center console.