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How do I boost my antenna strength on my 1999 Buick Century? - good hidden antenna car

I hid one of these antennas in my 1999 Buick Century. (I think someone told me that he is at the top of the rear window.) How can I increase the signal strength? Is there a product from the market when can I install? I lost radio stations in places where I lost when I drive the car with my wife. Thank you!