Check your V24 system CD. Check it burned as mode2 (use NeroTools or ImgbBurn). Check it burned with no errors. Mount image in a virtual drive and check all files in your DC against those in the image.

Eject and withdraw any disc from your navigation unit. Switch your car off and withdraw the key. Open your trunk and gain access to your battery. Do not open or close doors. Do not use your remote. Wait for all the systems to shut down -it may take some 5 or 10 minutes. All interior and trunk lights should have gone off and more specifically the navigation unit LED should have gone off. Disconnect ground cable from your battery once you are sure all systems have shut down. IMPORTANT: Disconnecting battery while MK-III is still shutting down may permanently disable the unit.

Wait for some 30 minutes. Re-connect your battery. Turn key to pos 1. Wait for all systems to boot and stabilise. Wait for your screen to fill and display whatever it does display. Check if you still have no navigation in your main menu –just in case.

Now insert the V24 disk and wait. Do not touch the key. Do not open or close doors. Do not touch the car nor your remote. Wait for hours if need be until the CD will be ejected on its own and the screen will ask you to press OK (right button). When that happens, remove the CD and press the button. Then wait for the system to reboot. Do not touch the key until it has rebooted.

If all fails: See if you can test your MKIII unit in a friend's car or try a friend's unit in your car. Eventually check that I-Bus wire is not cut.

I really can not think what else can you do.