Welcome to nico!!!
First might I suggest if you don't know exactly what your doing then get someone else to do it for you...
But the best way to do it would be to have the wires come down through an exhisting seam on the roof (apply silicone) then go down to either the battery or the fuse box.
Only thing is it will look cleaner if you put them into the fuse box in an auxillary spot then run them to a switch where ever you wanted it mounted. But again if you don't know what your doing I suggest get some help and watch them do it then learn from your helper
JUST MAKE SURE YOU PUT AN INLINE FUSE that way your safe and have no potential fire hazard
Good Luck.