In my work van that has factory fitted sat nav the gps antenna is stuck inside the windscreen above the rear view mirror (not sure why Iv got a rear view mirror as I can't see out the back!!).
When i had the screen replaced due to a stray scaffold pole the guy who replaced it said the screen was a special heat reflective type and the gps has to go in that particular place because that part of the screen is not coated and therefore would'nt interfere with the signal.
Do the early GTI's have heat reflective screens?
Oh and my gps antenna in the golf is just under the top tray and I get 10/11 out of 12