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dont all the factory nav's have the THX Package?
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Yes, they do.
Things you need at a minimum to add factory navigation are:
Nav head unit
Nav DVD drive unit
Sub-woofer amp
Four channel amp
Multiple wiring harnesses
GPS antenna
If you want voice control (factory systems have this) add:
Auto dimming mirror with factory microphone (used only on nav equipped cars)
If you want the steering wheel buttons to be labeled and work correctly add:
nav option steering wheel button assembly
If you want the full THX sound add:
THX door speakers (4) and subwoofers (2)
Advantages to factory nav (compared to AVIC):
Reliable (except for CD changer)
Fast boot up
Always knows where car is pointed even if not moving, or moving slowly
Easy to use interface with big touch buttons, few menu levels to traverse
Advantages to aftermarket nav:
Works with iPODs, MP3s, USB, ...
Much easier to install
Cheaper for newer maps
Some can be customized