Probably no help from me here, but ...
I have a '01, build date 6/01, with the Nav screen, etc.
When I first got the car 2 years ago, I looked into the aux/bluetooth option (of course) and concluded that:
1. Be very wary of the array of possible solutions presented to you - it matters A LOT exactly when your car was made. Something easy for a slightly newer car may be an engineering project for you, depending on your build date.
2. For me, it was going to be an expensive engineering project that I did not need, so I basically punted and got a cassette adapter. The aux cable comes out of the cassette thing, folded behind the nav screen. 6-cd changer in the back still works.
3. If I were ever going to do something on my car (unlikely) it would probably involve an Office-Space-style beatdown of the Nav system.
Wondering - why do people want to keep the NAV? I find it to be absolutely useless, and a bit of a hassle that I have to press two buttons every time I drive to shut the thing down. Is it that getting rid of the NAV will cause other problems with the car? Because I can live without the fuel consumption calculations, etc. And the GPS navigation interface is painful to use to the point of being painfully useless.
https://www.youtube.com/watch?v=N9wsjroVlu8