If you want to enable the Aux-in lines on a Sony headunit with UniLink, the simple and easiest answer is get the XA-300 from Crutchfield (or anywhere else you can find it).

If you're a cheap bastard like me who enjoys hacking together things, there is a better solution at http://gnunilink.sourceforge.net/. The most expensive part is a $30 1m Unilink cable (Sony P/N: RC61). You have no choice as the connector for the Unilink bus can not be found. I'm just cutting the cable in half and using each end for my 2 RX-7s. The rest of the parts can be obtained for ~$10-15 USD.
A great resource for reverse engineering the unilink bus can be found at http://www.cus.org.uk/~cleggy/.

Hope this helps; let me know if I can help.
Cheers!
Dave