Metallic will pose no problems for you.
the antenna for terestrial radio is in the rear windshield facing out, so metallic behind (or underneath) it will not affect your radio.
GPS antenna is in the front dash, so metallic on the sides will not affect this. If you intend on having your front windshield tinted, make sure to use ceramic here only if you have NAV. if you do not have NAV, then metallic on the front will suffice.
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[CENTER]^ that honestly shouldn't freaking matter if it's hotlinked.