Well, both ways are right. The better of the two would be to use the Pac. This will drop the high level output of the deck to low level input into your factory amp.
If you wire the BMW harness to your factory amp, you will notice that your system will distort quickly and the volume will jump to a higher volume more quickly than previously with your old radio.
A way to skip using the Pac would be to use your radio's low level outputs to the BMW harness. THis will provide a low level output to your factory amp. This will yield the best sound quality.
Find a set of RCA cables and cut the ends off it.
Then strip back the insulation on each rca end to show the internal wires. You will have a center wire and a outwire that "wraps around the center wire"
Strip back the center wire and tape off the rest of the wires.
Connect each of the 4 rca's (newly stripped) to each input on the harness (white, grey, green, purple) using only the positives. tape off negatives.
Then connect power, ground, and remote turn on lead to harness, taping off ALL speaker outputs.
Connect your power harness to the radio and your rca's to each channel respectively.
white=front left grey=front right green=rear left purple=rear right
This should do the trick and should sound great!
Greg Mannino
Car Audio.Video.Security.Navigation Guru
Electrical/Mechanical Specialist