There are numerous threads (SEARCH) that go into lots of detail.

There's no free lunch here. All the different approaches that involve using a factory head unit are going to have you spending the same money and more time to come up with a solution that is no better than just ditching the factory unit and plugging in a unit that does what you want out of the box. I fact, it may actually be no better than what you have going on with the FM modulator...

In short, the head unit slides out and unplugs from the harness and antenna, the new head unit plugs in and slides back into the opening.
The tool to remove the head unit costs less than $10.

BUT... The nav unit is going to involve a few more connections because you have a satellite antenna and for the CD changer, those. Since they're not in the car, you're going to have to figure that out from write-ups you will find on this forum. My guess is that if you're asking questions about using the non-Nav unit you will probably find it difficult to do the necessary stuff without help from someone who's done it before.

Honestly, get some opinions about the NAV unit. Remember, the stereo isn't any better, and the NAV capabilities are (sorry guys) pretty limited by comparison to moderately priced hand-held units (that are infinitely more flexible and allow upload/download of GPS data).

Personally, and this is just my preference, the only thing you're getting with the NAV unit that you don't have with the non-NAV unit is a CD changer connection and an entry-level built-in GPS. Oh yeah, it "blends" with the look of the console. It wasn't enough for me. I wanted way more stereo than the NAV unit offered, and way more GPS than the NAV unit had. Not a good value proposition for my needs.