If it's the same as the 2003 model, it should be 86170-34020. I bought one off eBay but it doesn't work with my car because mine has navigation. My local Lexus dealer tells me the part number of the remote for RES w/navigation is 86170-60030.
If you have one of these remotes, please post a message or a pic of the back of the remote showing the part number. Thanks!