From my experience, its the white wire. It should be a smaller gauge wire, probably a 22 gauge wire.

I've never seen an X5 without an amp. I have an 04 3.0i with no dsp and no nav and it has a amp from the factory.

If your x5 does not have an amp indeed, then go with the white smaller gauge wire. You should be fine as the amp will turn on and off with your ignition switch. If you run into problems with a popping or clipping sound from the sub as you turn your vehicle off, add a relay to the remote wire.