Im not a full expert on this so if i am wrong anybody please feel free to correct me.....I believe you need a headunit capable of having an Aux IN socket on the back, that way you can connect the red and white Aux wire's that comes off your PS2 into these and the yellow cable for the picture into the TV (where ever your gonna mount it :S lol.) via the discounnectable scart socket which comes when you buy your PS2
When you are using your PS2 you will switch your headunit over to Aux 1 (or whichever socket its plugged into) and the sound will now be played through your car system
Hope this helps
Correct
You need to have a headunit with either:
a aux in 2.5mm socket (headphone jack) a adapter will be required for this option
a set of aux in RCA Connectors,
as said above the audio connections coming from the PS2 (red and black) will go into the connection on the head unit and the remaining connector (yellow) witch is picture one goes to the TV's input, probably will need to use the scart adapter unless the tv has RCA inputs, witch very few do.
hope that clears everything up
if you need a link to the adapter i mentioned above just shout up and i will find you one.