There are no regulations prohibiting stickers on side windows or back windshield, however it is recommended to keep it in line with Florida window tint laws.
The percent of visible light allowed through your car windows is called VLT: Visible Light Transmission. The percentage of light allowed through your film and glass in Florida is very specific and different for sedan cars and SUV cars or vans.
Vans/SUV's
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Back & Side windows: Must not be more than 35% reflective.
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Back Side windows: Must allow more than 6% of light in.
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Rear Window: Must allow more than 6% of light in.