I didn't have to remove any of the other trim, just the 'letter box' surround for the DVD unit. Like most of the other parts of the trim it unclips with careful insertion of a flat bladed screwdriver.
Pull out the 4 connectors from the rear of the JDM unit. Only 2 will fit in the UK unit, so you can't go wrong, then just tape up/tuck away the redundant ones. Slide the new unit into place and refit the screws into the side mounts (screws are a bit of a bugger to refit). Clip the trim back in and your'e all set to turn the system on. Can take a few mins to boot up.
You may need to use a programme CD before installing the DVD map disc if nothing has been left on the replacement unit.
I replaced mine almost a year ago for a 2005 X-Trail unit and have had no problems.