http://wiki.phathack.com/Versions_and_Compatibilityhttp://www.phathack.com/pinouts.htmlhttp://wiki.phathack.com/Cable_FAQhttp://wiki.phathack.com/Hardware_FAQThere is some info for you to digest before posting this question.
With that out of the way, there are some headunits you have to have a more recent hardware revision in order to get a phatbox to work with. (some japaneese cars if I recall correctly but I don't remember which ones.. someone please PM me with that info and update the wiki page
http://wiki.phathack.com/Versions_and_Compatibility for it if you would.. I'll update this thread.)
Generally you can take a VW phatbox and turn it into anything except for these newest supported headunit phatboxes. You can make a keg into anything a VW phatbox will hook up to as well.
You need the right cable for the head unit connection from the phatbox and you need the correct firmware loaded onto the phatbox / keg and it should work.
If you have general stereo related questions I suggest heading to a general car audio forum for that stuff.