I am by no means an expert but I did install the oem cd changer in my 2005 X5. The wires should be tucked underneith kind of shoved down under the floor in the back left corner panel. If you search on youtube for bmw x5 oem cd changer install you can find tutorials that will show you exactly where they tuck the wires away to if there is no cd changer and what they look like

hope that helps