2008 nissan versa, stereo, head unit

I bought a 2008 Nissan Versa 1.8S hatcback in July, and my wife and I have almost no complaints. What we've really wanted, however, is an aux-in for the head unit for my iPod Touch or her iPhone, which only the more expensive model comes with. Since that was the only thing we really wanted, it didn't make sense to pay $2k+ extra.

I realize that there are lots of aftermarket options, but I'd like to keep the stock look (and not void any warranties, which this probably would, but it would take a pretty attentive employee to notice this stealthy upgrade). I found a few of the factory head units from the nicer car for sale, however, I'm wondering if the 6-speaker head unit will just plug right in to the 4-speaker car's system - this is the big issue.

Here are two pics; the first is of the rear of the OE 4-speaker HU (like in our car), and second is the rear of the OE 6-speaker HU (which is the high end sound system HU):

The only difference I can see is that one extra plug, second from the left, on the 6-speaker HU. Someone told me that this *should* work IF there's no external amplifer on the high end Versa sound system, and IF the high end system is NOT Bose (which it's not). I don't know if there's an external amplifier for not, which is the big issue I believe.

Can anyone help?