I agree with brian, "The key is to make sure that the MKIII has fully shut itself off." I have done the swap, (mk3 to mk4) I just waited for the red led to go ot then swapped over the units, I did not disconnect the battery, but thats just me.

My unit asked for a OS disc as soon as it powered on, (it was factroy fresh), so it might be an idea to get hold of an OS disc (v29 will be fine) while you do the swap.

If your unit just powers up, then you should be good to go, just put your map disc in and away you go.

Your map CD will work fine in the mk4, but like brian says the DVD holds more data.

If the unit is 2nd hand you may also need to calibrate the mk4 once it's installed, this is easy.

Find a motorway (freeway) and travel for around 15 minutes at a constant 50 mph, this will the re-calibrate the unit to your cars speed sensors.

Once your mk4 is up and running turn on 3d perspective (as above) and enjoy.

HTH

Merry Christmas.

Oh, BTW, it might be an idea to recode the mk4 once its installed, use Jochens navcoder to do this and it will setup the mk4 for vehicle VIN, colour etc.

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Sept 03, M3, Silver Grey. Retro-fits: Modded Mk4 DVD Drive, OEM Bluetooth, VR Control, Rainbow Speaker Upgrade, Sony SOT Amp, HK Sub, CCFL, Intravee II, NightMode.