Originally Posted by KiwiJochen

You need:
screen (4:3 or 16:9)
radio module (business or professional, old gen or new gen)
video module (also known as the tv tuner)
cd changer (option)
amplifier (option, standard or dsp)
telephone (option)
tv antenna amplifiers (for TV)
retrofit wiring loom (better: complete retrofit kit)

It is only a very small extra to add navigation...