2009 Xterra Stereo Upgrade?

Hi there!
I am looking into purchasing a new 2009 Xterra SE. Inevitably, I would like to upgrade the factory audio system.

So, I went to Crutchfield.com and went through their 'Outfit My Car' thing (I selected the model without the Rockford Fosgate system, but factory installed), but to my surprise, when the recommended head units popped up, it said none of them would fit my car.

Next, I went back and changed the option where it asked if the car had the Rockford Fosgate system or not, and I selected 'WITH Factory installed Rockford Fosgate System'. The results now showed that I could fit an aftermarket Head unit in.

Now, I am quite puzzled as both these units are factory installed. Can any of you guys please clear this up for me?

Do I have to get the Rockford Fosgate system in order to put in an aftermarket head unit? Or can I just get the bottom of the line factory installed system and still be able to put an aftermarket head unit in?

Thanks!!
Wirercx

PS - I know the Fosgate system is free for a limited time for the 2009 SE, but I would just like to know what the deal is here for arguments sake AND in case I do not end up purchasing the car until much later.
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