Harness, the replacement harness is plug and play and therefore easier than the retrofit harness but you have to either open the door to connect the speakers or cut the existing wires near the doors and make the hook-up.

You will also need:
Onboard Monitor with Cass player
Mk4 Nav computer
OBM Radio which goes in the trunk/battery space
A CD changer or other device that will interface music media
GPS antenna
DVD with Map software
+$150 for dealer to reprogram the ECU to acknowledge the OBM set-up

You will need to:
Remove the left trunk covers
Trunk floor
Rear seat cushion (not the seat back)
Front seats
Console
Rear carpet can be lifted out of the way
OE Bizz radio

I think none of the after market set-ups look any good once installed and so I went with OE. But I do also realize this hardware is pretty ancient in design and so it really depends what you want from it. Our Xs are commuters/school bus for the most part. So minimal Nav ability and some kind of music reproduction is enough. Others want all singing, all dancing Nav and opera style music hall.

At the time I created a longish thread in the NAV section of the board.

Good luck.

__________________
2003 4.4i black ext./black int./black headliner (kid's runabout)
2014 535d X-drive, M package, silver/black interior

Sold but not forgotten:
2009 E70 35d, black ext./black int./black headliner (sold 2021)
2006 4.8iS Le Mans blue/cream int./black headliner, SOLD in 2012 sadly...

Other hardware:
2015 Cayenne S (wife's new DD and definitely wroooommmmm)
2016 Tundra 1794 edition crew cab, luxo hauler
2005 Tundra crew cab, weekend hauler. Sold after 150k very happy miles.