After installing the Bluetooth ULF module you will need to mount the Bluetooth antenna. Most people install in the E38 C-pillar or behind the rear seat. This process is quite simple and detailed in the installation guide that ships with your Bluetooth retrofit kit. You can just run the other end of the RF cable connected to the ULF module thru the c-pillar access grommet to the right of the CD changer. This grommet is where all the BMW wiring passes from the rear electronics compartment to the cabin of the E38 (running to the c pillar and behind the rear seat). Just remove the rear c-pillar, push the wire thru grommet and retrieve the wire on the other end.
After you fish the Bluetooth antenna wire thru the C-pillar, mount the Bluetooth antenna with double sided tape next to the other electronics. Find a location that is not raised higher than any of the other components, as you will need to put the C-pillar cover back in place.