I have a 2000 BMW 540iA with Premium Sound DSP and Navigation, so I wanted to find out what all the wires for all 3 connectors to the DSP amp were, well thanks to Dwayne Mosley (from bimmer.org forum) for providing this information to me. 

2000 DSP Premium Sound connector information

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

The pinouts for the 18 pin connector are:
PIN - CONNECTION
1- Not used
2- Low freq. negative (-) signal - Transceiver/charging electronics
3- Not used
4- Not used
5- I-bus Signal Line - Splice X18344
6- Not used
7- Not used
8- RH front speaker - Connects to Radio
9- LH front speaker - Connects to Radio
10-Not used
11 Low freq. voltage (+) signal - Transceiver/charging electronics
12-Not used
13-Telephone mute signal - Transceiver/charging electronics
14-Not used
15-Telephone ON signal - Transceiver/charging electronics
16-Not used
17-RH front speaker + - Connects to Radio
18-LH front speaker + - Connects to Radio

The pinouts for the 26 pin connector are:
PIN - CONNECTION
1- LH front woofer +
2- LH front woofer -
3- Not used
4- RH front mid-range -
5- LH front mid-range +
6- Not used
7- Not used
8- Not used
9- RH front mid-range +
10-LH front mid-range -
11-Right rear mid-range speaker/tweeter +
12-RH front tweeter +
13-RH front woofer +
14-LH front tweeter +
15 LH front tweeter -
16-Left rear mid-range speaker/tweeter -
17-Not used
18-Not used
19-Left rear mid-range speaker/tweeter +
20-Not used
21-Not used
22-Not used
23-Not used
24-Right rear mid-range speaker/tweeter -
25-RH front tweeter -
26 RH front woofer -