Rear speaker kits replace the two cubbies with four 3.5" speakers, two on each side. Cars that did not come with the kit are not wired for it, and the instructions are available online. Basically, once you install the speakers, you run a wire under the passenger door sill, and can either connect it to the head unit for 15W or upgrade to a 6x40W 996 amplifier and run it to the trunk through the firewall.
There is a ton of info on rear speaker, door speaker, and amplifier upgrades online, and if the kits are no longer available download the parts manual PDF from Porsche and order the parts directly. It is also important to note that the Bose rear speaker setup is not compatible with non-Bose audio systems, so you can't plug and play a Bose rear speaker setup. You have to install the regular rear speaker upgrade or change the entire audio system to Bose.
Best,
-Rick
