If you wish to upgrade ROM, the BT Stereo functionality is already enabled.
You will need two files:
ftp://ppcgeeks:ezupload@up.ppcgeeks....KU3.5+_pr4.rar
ftp://ppcgeeks:ezupload@up.ppcgeeks....5+_pr4_SP1.cab
The first one is the latest upgrade to 3.5
The second one is a patch for it.
Download them and carefully CAREFULLY read (at a minimum) the first post in this thread. It's updated fairly often, so come back later for more details or clarity.