You drop the rear of the headliner and there is a 17mm nut on the underside that you undo.
You then need to route 2 seperate RG174 coax's from the head unit and bluetooth module to the undersde of the aerial(Only one Coax if you are just doing the GPS) and place a femal Fakra connector of the correct colour on either end.
Oh and as for the route you go from the dash, along the sill panels, into the side panel in the boot and up the c pillar into the roof.