Don't believe them. There are a lot of writeups on DIY rear speaker installations in this forum. Just do a search and you will find many. Is your Boxster a 986 or 987?
My understanding is the amp in the frunck is connected by fiber optics, from there it should be regular speaker wires. You may have go run them but should be a fairly simple diy.
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Hi my car is a 986 boxster s a 51 plate it has a Becker and semins PCM with sat nav and phone built in and when I put it onto settings it comes up with the fader that goes from front to rear so really can't understand why it wouldn't work even when Porsche says it won't unless I spend about £8000 to get it rewired I have never heard of a stereo having to get programmed to work rear speakers cheers simon