I mentioned in an earlier response that my dealer had printed out the schematics for my '00 540i DSP system. I had several requests to post them here for sub hookups.
The pinouts for the 15 pin connector are: PIN CONNECTION 1- Sub 1 + 2- Sub 2 + 3- LH Rear loudspeaker + (Touring LH rear speaker ground) 4- Radio on/antenna signal 5- RH rear loudspeaker + (Touring RH rear +) 6- Sub 4 - 7- Sub 3 - 8- Terminal 30 connects to Fuse 56 9- Sub 1 - 10- Sub 2 - 11- Touring LH rear + 12- Touring RH rear - 13- Sub 4 + 14- Sub 3 + 15- Terminal 31 connects to Ground Point