No, you can use the factory harnesses, you just have to splice into them at the stock amp's location. But this is really only useful in the case of the front speakers, since getting wires through the front doors is a bitch.
If your current stock system is the standard Nokia 10 speaker setup, let me know which wires you need to identify on the stock harnesses. I have the same year 325i, and I went through all this crap earlier this year
