The '-' means that the Navigation computer is not seeing the GPS antenna, which 99% means the antenna is broken (and 1% the navigation processor itself)
You can either replace the glass mounted antenna or put in a new one under the parcel shelf (above the metal below the material) and connect to the back of the navigation computer in the boot. Easiest job is the latter, see here ..
Comand Online Ltd :: COMAND Retrofit parts :: Genuine MB GPS Antenna and adapter for DVD and Hard Disk COMAND APS
Richard