Agree with Rich, above, cabling up roof rack mounted accessories is a PITA, especially to devise something that's not exposed to snagging while offroad.
I used a single waterproof multi plug, through the roof, as close to one of the rack legs as possible, then cable-tied the wires underneath the rack to avoid snagging.
Can't do right for wrong IMO, but some work out better than others. No problems with mine up to now. I do suffer a lot from overhead branches and stuff, so maybe I was being over-protective.