You can always stick it on the plastic roof inside. Works just fine through the poly.
They will work inside (bottom of windshield is possible too) but work better outside. Had this problem with our Saturn; finally found a spot just above the door where the magnet would hold. Not really an issue as the car is somewhere in smart limbo.
My wife drives an all aluminum Jag. for years we have attached the XM antenna with some of that yellow tickytacky office stuff. It works great even in a car wash. AL
I have Sirius in my TBird, which is mostly fiberglass. I have the antenna velcroed to the top far right of the dash and have no problem with reception.
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