Theres other ways to do this, and retaining the original sat nav screen.

Get any DIN E size front removable panel CD system, fit in boot and extend the cable for the fascia to mount in front of vehicle, utilise the pre outs for new amps and speakers, then hard wire the OEM sat nav to a telephone speaker under dash.

This way you retain the OEM look of the front still and can utilise the OEM sat nav etc, but have a nice sounding aftermarket system.