First did you check what type of connector you have? You may have to unwrap the black sticky tape wrapped around the box connector to look at the connector and wires. If you have the 12 pins connector as described in one of my previoust post you will be able to feed the high level inputs (speaker level inputs) of this amp from pin #5 (+) and #6 (-) of the stock amp connector. You will of course need to connect the ground wire of the high level input to a ground somewhere in the car.
If you have neither the 15 pins or 12 pins connector described upper in this thread the only other one I am pretty familiar with would be a 42 pins connector that are found in other BMW models equiped with Harman/Kardon audio like some e46's for example but which I have not seen on an X5 yet.