When everything is unplugged. Turn on the ignition and plug the HU back in at the dash.
The plug in the tuner and wait 2 mins.
I have a the same year X5 with nav and DSP and this worked perfectly for me.
The DSP on the HU greyed out after a couple seconds then disappeared soon after.
I think the HU was powered off when I did it. ( not 100% )