Hello I am looking for some help with regards to compatibilty and hardware needed to install subwoofer and Amp.

I have a friend who has DSP OEM sub in stealthbox with stock amp. I currentlu purchased a 2004 X5.with navigation system and 6 CD changer. My friend only has the sub and amp with no harness. Can someone let me know what harness would I need and where I can get it from. I am assuming I can go to enfig but if anyone has other suggestion please let me know. thanks in advance