Been there, done that. Not very difficult but the seats need to come out and either replace the wiring loom with the original wiring loom for a car with NAV or run the retrofit loom. Both need to be run front to back so no big difference. I opted to replace rather than add the retrofit loom. Then you have to sort out the GPS antenna in the rear spoiler. There should still be a thread in the electrnics forum on that with a lot of pics.
http://www.xoutpost.com/electronics/...s-harness.html
I had the car coded at the dealer, the SA simply siad: "you've been busy...."

For me it was worth it back then, now I think technology has moved on too much and there are reasonably good after market units. The problem remains what to do with the need to control some things though the MID.

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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.