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You can find PDF owner's manuals floating around online, you can purchased used on eBay, or you can order new from a dealer or part supplier. A part number is hard to pin down by year, but the PN should start with: WKD.986
There are several single DIN nav options, however, if you want to keep the OEM look take at look at the Becker models. I can't comment on upgrading the dash speakers, but adding rear speakers does make difference, they help 'fill in' the sound. Not aware of any direct plug and play amps, besides the 6x40w 996 amp, which would only be helpful for adding rear speakers.
Porscha
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