Piece of pi55...
Take the old unit out, slide the new one in, pop in the security code (which I hope came with the unit ?) and you are good to go. You only really need to have it coded if you have the highline display, as the NAV instructions, compass arrow ect. won't show up unless that's done. Also, it will throw an error in VCDS about the unit being incorrectly coded, but it doens't affect the operation of the unit or the car in ayway, and if you don't have VCDS, you will never see the error code anyway
If you only have midline dash, then don't even bother coding it, just fit and play.
Just hope that whoever fitted the aftermarket unit didn't hack into the original harness and quadlock connector and everything is still in place ?
Just remember the limitations of the MFD2, which is that the NAV disc has to be in the unit when ever you want to use the Nav function, meaning you can't use the a CD for music, therefore only radio will be available unless you have a 6 disc changer or Ipod prep. That's the reason why ALL MFD2's were sold from the factory with either a CD changer or Ipod prep fitted. TBH, if you have either of the former then it's no big deal but if you haven't it will drive you nuts.
Don't get me wrong, for less than 200 smackers, it's a really nice unit (especially if it's the later DVD drive version), but just be aware of it's limitations. One of the reasons why I upgraded mine to an RNS-510 with MDI.