Here are the rear speaker installation instructions, I bought the kit and it just plugs in. A lot of people buy the speakers and make the mounts, grill and wiring themselves:
Pelican Technical Article: Installing the Boxster Rear Speaker Kit - 986 / 987
I replaced the headunit shortly after with a Pioneer 940BT satnav unit. This meant replacing the rubbish MOST Porsche amplifier with an Alpine unit which increased the bass from the door subs immensely. I also replaced the dash speakers with Alpines. I got a good price for the CDR23, amp and CD changer which paid for half of what I spent:
