I did this a while back, but it was enough of a hassle to warrant posting 3 months later.
For the uninitiated: on the BMW E46 models with OEM Navigation, the original stereo and amplifier are located in the trunk. This means for aftermarket installs, you need an absurd amount of wiring to travel almost the whole length of the car. A good part of the interior needs to come out as well.
But once you get over the growing pains this setup is actually quite nice because it saves a ton of room behind the dash. It’s definitely a trick to get things sorted however.
Here are the photos I took throughout the process: