The rear speakers install inside the rear shelf cubby, most Boxster's have them, but some of the early years did not. (
You can see a picture and install instructions here). You can power the rear speakers from the Becker HU, but will only get 15W, if you want more power you need a larger amp. I used a 6x40W from a 996.
There is a lot of information on rear speakers online, if you search for '986 rear speakers'.