Hi, hope this helps i found this when i was looking to do this to my car too, Im not sure if you need a hi input amp or not but this shows in detail how to add RCA sub outputs.
http://www.lexusownersclub.co.uk/knowledge/index.php?page=index_v2&id=20&c=14
Harry
Thanks everybody, Yea i have seen a couple on the site but they all the wrong codes, Does andyone know if it has to be the same code or will it work if the cd changer is in the same place, top or bottom?
Thanks