First off, with the BT/Assist TCU, the idea is that the external antenna (shark fin / bumper antenna ... whatever) is shared between Assist and Bluetooth calls (when the handset is plugged into the cradle).

From what I figured out on my 04 (be it a M3 with the shark fin) the cable incompatibility was caused because there wasn't a correct cable that ran from the center console back to the trunk for BT handset antenna signal. On the 05's that cable is there. 04's ... is not. There is however a old style CPT9000 antenna cable there ... however I never figured out how to make it work, so gave up. BMW ofcourse were no help, since it wasn't supported, and they dont plan to support it. So for powering my handset I just use the glovebox torch light adapter. The handset (SE T637) works fine and doesn't seem to need a external antenna.

If you make any progress with the cradle and your '04 please share it with the rest of us ! :-)