are you just wanting to add the Porsche /Becker 6 CD changer to the Porsche/Becker CR21 (CR210 here in N. America) ????
i take it you have a '96 and up 993??
(If you have an early 996, some of the following details may be a little different.)
EDIT: saw your post with the picture of your 993...my question answered
If this is your scenario, install is quite easy. Basically 'plug and play'
You will have to purchase the wiring cable from Porsche that plugs into the back of the head unit and goes to the CD changer. This is the only wiring that is involved! I think the cable is about $80. You will also need the metal bracket that mounts the CD changer in the front luggage compartment of a 993. About $15.
I am not sure what the CD-changers sell for here new; but they usually sell for $200 to $250 used and on ebay. ( cable and proper bracket not included most often)
Here is a DIY that i contributed a few pictures to:
http://p-car.com/diy/cdchanger/ It includes the part nubers for the mounting bracket and wiring cable/harness.
max