Yep, the Rectangular cage will fit into the stock cage on the Mk4. You will need to cut out the plastic rear support of the stock cage, as the new radio will be deeper than the stock one, but the rear support is plastic and won't take much to remove.
As for the wire harness, I am pretty sure that the MkV uses a compatible harness as the MKIV, if not order from Enfig.com.





