What do you currently have? Mine is the 2003 model which is a pain as it had the fibreoptic MOST CDR23 head unit. I had to remove this plus the amplifier and CD changer. I installed a Pioneer 940BT satnav and is a huge improvement in capability and sound quality.
I replaced the dash speakers, cut out the Porsche speaker and used the ring to hold an Alpine speaker. I bought the rear speaker pack and grills for the rear storage, this makes a big difference. If you dont have them, get the Porsche door subwoofers - always some on ebay. I have the head unit running the dash and rear speakers and an Alpine Amp where the Porsche one used to be running the two door subs. I took my time so apart from the head unit, it looks very OEM.
It is easy to change the speakers yourself. The rear speaker kit comes with all the wiring so you can easily attach this to the head unit or front amplifier if you have one. There are guides for this plus how to replace the dash speakers, I can send you them if you want to try. The dash speakers are very expensive to replace so I recommend you do what I did. The head unit is well worth replacing and I would definitely get the subwoofers if you dont have them fitted already.
