The Sat nav I have in mine is a VDO Dayton unit, Mazda did a run of RX8s and MPS6s with them in (about 40 of each I believe). So mine has a VDO Dayton PC5600 under the passenger seat, then a Mazda screen where the map compartment is on a standard 6. Parts prices - the screen is about about 8K from Mazda. Yes, I said 8K. E-I-G-H-T. I know this as they have had to replace mine twice under warranty.
. It's a sweet setup as Tricache says, but it's strictly sat nav only not audio. You can extend the VDO system with AV and Bluetooth capability, but it's not cheap, and there are better options I believe. I've ended up going Parrot for Bluetooth, and iPhone > Xcarlink adapter > stereo rear input on mine. So i have a much better end package than stock, but I haven't played with speakers or amps.
I am told that putting some decent sound deadening in the doors makes a difference.