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I've got one in my truck, no speakers in it though. Speakers up there and behind you destroy the sound stage, so I never recommend putting speakers up there. I've got a CB where the radio would mount, and I'll eventually hook up an external speaker to the CB and put that in one of the speaker holes, and cover the rest up. They have a neat little flat spot facing the driver right over the visor, great for mounting switches.
It doesn't matter what the speakers are or how you wire them, putting them behind you or even with your head will trash the soundstage. Speakers should ONLY be located behind the driver when being used for rear fill, and they should be limited to a lower midrange/midbass set if possible.
I've got one in my truck, no speakers in it though. Speakers up there and behind you destroy the sound stage, so I never recommend putting speakers up there.