Before you consider the final location, use tape to hold it in place so you can test it out first, it will work this way until you figure out the right spot.
The SunPass sticker is very picky too, the readers are installed very high, the sticker needs to be read through the windshield unobstructed, but it might be possible to install it on the dash cover toward the front (if that's a better option).
If while you are trying a better location, the SunPass isn't read, it will get billed to you as long as your license plate is registered to your SunPass transponder, even if you get the SunPass letter, all you do is call them and tell them your transponder number, you won't get a ticket for this like they used to do years ago, but you want to get it fixed.