Sammy do you have DSP?
If you do, you can't get one of the aftermarket Nav units in there without ripping out the DSP amp, replacing with aftermarket and re-wiring the speakers as far as I can tell.
If you don't have DSP, you are lucky. And if you want my opinion (and you know we both think alike) I would get the newest version of the Dynavin unit, it looks the most factory out of any of the aftermarkets at about $950 as far as I can tell. Comparing to the Hualingan unit is not even close, the Dynavin looks the part by a mile.
Now don't get me wrong many here love the Hualingan and say its a faster unit etc, but if you are like me you want the factory look and can sacrifice a bit of performance, I would go with Dynavin.
However, if you have DSP as I do, we are basically out of luck, because by the time you get a new amp, re-wire the speakers, etc, and then hook up a Dynavin, you're probably close to the price of retrofitting an entire OEM Navigation system.
www.bimmernav.com if I recall correctly, they source all the parts for a complete OEM Navigation retrofit with MK4 computer, it costs $2800, so obviously its not cheap, but if you have DSP it's basically the only way to go and keep the DSP unmolested.
I might be missing something here, but from what I know it's not an easy process to get rid of DSP if you have it.