Extreme noob question

[Socialsin]

I have an '06 GP Base model sedan and I want to add a double din DVD/CD/NAV/SAT/WIFI head unit.
My question is this: I went to the website www.the12volt.com to look up the audio specs and was told I'm going to need two separate "Interfaces" for the head unit and also for my steering wheel audio controls.
I went to www.metraonline.com and saw 15 different interfaces for my specific GP.
Long story short which damn one do I need?
There is only one for the steering wheel audio controls, so I ordered it, but which of the 15 interfaces do I need for the head unit?
Better yet, can some PLEASE give me a list of everything I'll need to install this head unit.
This is the model Pioneer AVH x2800 Navigation unit.
Thanks everyone in advance for the help.

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[hotdogstand]

This link hopefully works: http://www.crutchfield.com/p_130X2800BS/Pioneer-AVH-X2800BS.html?search=AVH_X2800&skipvs=T
Anyway, you'll need one harness to convert the new head unit to the factory plug. Do not cut and splice the plug as you'll lose some features (door chime, etc) and unless you are a pro (and I'm assuming you aren't because you're asking questions about parts) then you won't do anywhere near as good a job sealing those electronics back up.

The steering wheel harness is separate and allows you to retain full use of the factory steering wheel controls. I'm sure I'm supposed to support vendors here, but I just got here and don't know what brands support the forums. I have used Crutchfield before for installation gear with very good results.

EDIT: checked the link... to the right of the product image you'll see "Fit info for your pontiac" and "see the installation gear" <--click that and it will give you two options to get the harness(es) that you need for your install. Good luck