Upgrading from the Symphony II to the Symphony II+ from B7 vehicles is not only aesthetically pleasing, but also allows for an AUX input! Sound quality is better(subjective), but radio frequency quality is lowered.
The Symphony II+ ranges in price from $150 to $300, used. Seen them go for $150 without the four digit radio code, which means you need to goto the dealer to unlock it for $90 bucks. I managed to score mine from ebay for $90 with codes and two scratches... They are un-noticeable to the untrained eye.
You can make your own harness with the following:
Quadlock = $16.99
C Block Connector = $18.73
AUX Jack complete with B Block Connector = $14.99
Antenna adapter = $9.99
approximately $61.00 dollars for the harness.
Some vendors sell complete harness, but I have heard that the pinout were wrong and some modifications were required, hence why I built my own. About 70 dollars for a pre-made harness.