The GPS antenna on the X5 is located inside the spoiler. So, tint won't affect GPS at all-- it is already outside of the car. That antanna that looks like defroster wires is your car's "Diversity Antenna." That is for AM/FM/TV/other terrestrial reception. You did good to put non-metallic tint on the diversity antenna. Metallic tint kills AM reception and significantly degrades other reception.

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2000 E38 750iL Highline Edition, Upgraded with the lastest 2006 X5 Electronics
MKIV Navigation, 16:9 Display, Bluetooth, TV/Video with Backup Camera, Sirius, and Aux Input