Ok, this might get very technical, but only the very knowledgeable will understand.
Next to the BGA MCU are 2x64Mb Fujitsu TSOP48 bottom boot block Flash. Memory dumps on dead mk3 navis show corrupted (containing mostly 00h) sectors, checksums are vastly off. The entire 32k "protected" boot block is stuffed. This is why the OS CD does not load. Reimage the onboard Flash to fire up the unit. It's not possible to do this by the user interface CD. I leave it to your imagination how this can be done.