Yeah, you need to remove the NAV unit, pull the plugs on the back. Try reconnecting and see if resetting it makes any difference.
If not, the larger of the connectors on the rear will have two fibre optic cables entering it. You can remove a little module from the larger housing that has just the fibres in.
The small module has a blue tab, push this out and then gently release the tabs holding the fibres in place, be careful! Don't bend the fibres too much.
Simply push the fibres into the rubber tube facing each other and tape them to a stiff bit of wire to hold them in position.
This will allow you to use the rest of the idrive until you get a new NAV.