Take KiwiJochen's advice and get a copy of WDS. Print a copy of non-DSP wiring and then one of DSP wiring to see which wires to swap.
I helped a friend retrofit DSP in his 2003 E39.
Depending on where you get the amp from, it should be coded for your vehicle type (the equalization is different from model to model) and for Nav or non-nav (you will get sound for about 10 seconds, then mute if this is not coded correctly).
You will need to tap into the I-Bus so the car "sees" the amp and the radio module (BM53 or CD53) will output the correct signal level.