I installed mine, the way i did it was to remove the long 4 feet by 4 inches rear trim and but a 1" thick by about 12" square and put in between the head liner and roof, the hole should be about 4" to 5" from rear edge of roof and center it from both sides.
hope that helps..
If you have the A4 b6 then you dont need to remove the headliner. the rear light in the roof can be removed and you will see the hard board cut out (a circle about 3inchs) that is where the sharkfin is supposed to be located. mine went there and workd so much better than the internal GPS. tell your bodyshop to push a sharp pointed tool up in the center of the hole from the inside of the car this will give them the pilot mark to drill they should use a cone cutter to do it properly this will drill a series of holes at different widths as it cuts the roof. scary to watch but well worth fitting a sharkfin over the internal GPS arial. hope this helps.