Hi
I just installed a lighting bar to my disco, i ran the wires through a hole in the roof, down the side of the windscreen on the inside, the cover should come off. Then i ran it down through the dash to near where the fuse board is and then i ran it through the bulkhead to the battery. I Connected the earth wire to one of the bolts holding the lighting bar on and wired a switch into the dash. Drill a hole in the roof and get a grommet that the wire fits tightly in and force it into the hole, this creates a waterproof seal so no damp headlining!
Hope This Helps