Bestkits.com
you can search and find the parts you are gonna need, this is the site my local bestbuy used to figure out what parts i needed when i bought my new HU
i got a new HU and wiring for about 250, i also got a killer deck that has ipod control AND an aux input.. for one that will do just aux you can get one for 99, but ya get whatcha payfor.. and by that too, just cuz its the most expensive doesnt make it the best