Hi,
Bob dueb - that's actually incorrect! premium brand head unit manufacturers will have a steering wheel control interface they sell to use with their head units. There's also aftermarket solutions from the likes of PAC, Connects2 and Paser.
Fricchione - The issue with trying to interface new technologies with 10-15 year old designed systems is compatibility. Certain things can be done, and other's cannot. Those items which can be done, are often expensive in relative terms compared to other solutions which have a different direction. I'll explain a little more..
You sound as though you want to 'add on' to the oem audio/GPS system in the car. You have a 2000 model year E39, did it come with the 16:9 widescreen monitor / GPS setup? if so, you'll most likely have a Mk3 CD disk based GPS computer. The one most owners go for is the Mk4 DVD disk based GPS unit from the later E46 and E53 cars which plug n play in the E39 with the oem audio/GPS setup above. They do cost around £300-400 over here in the UK and the US pricing is likely to be around $500-700 USD. It's ONLY that oem setup which is able to have the oem Bluetooth module retrofitted as well, so if you have any other oem setup, you cannot add oem bluetooth. Assuming you do have the 16:9 monitor/GPS setup, then you'll need to find the last version of the bluetooth module which had the last 'Bluetooth Stack' - given that these modules will be from around 2005/6 from E46 and E53's they're coming onto 7/8 years old. New mobile phones might not be able to 'pair' and 'connect' with such an old bluetooth version, so perhaps the next round of iPhone 6 or 7 just 'wont work' - if you do find a module, they'll be perhaps $300-400 USD (plus any fitting charges) and you might need to buy looms. For an iPod connection, you'd need to look at solutions from DICE, Dension, mObridge, GROM - expect to pay perhaps $250-400+ for this.
As mentioned above, there are such things as aftermarket bluetooth hands free car kits available from the likes of Parrot, Motorola etc. They can be connected to the existing audio setup you have and can stream iPod as well. You'll have to mount a control screen somewhere in the dash though, so if you are more of the wish to keep the dash with uncluttered looks, you might not want it, but if there's no other way, you may well have to!
The chinese all in one replacement head units which have GPS, iPod and Bluetooth built in are a 'possibility' - the best of these are the Dynavin's and if you do consider this route buy from Jeff out of Fresno Calif. as he's the US official importer/dealer, his website is here -
http://jandtdistributing.com/dynavinsolutions/ - any of the other cheap chinese units, are more risky to buy as their quality is much lower with much less support.
If you prefer an easier life and want products that a high quality and work well, then consider getting a premium brand double DIN head unit fitted. Ipod and Bluetooth are pretty much standard features on most units out there. Those who want to 'add' a navigation module have the choice to do so - with software like Co-Pilot and TomTom available on Android and Apple markets at like $50-70 USD, many people use their smartphones for GPS duties.
Have a look around this forum and other BMW forums as much can be done to an E39, all depends on what you want, how much you'd like to spend and if you *must* keep the oem looks in the dash.. or not!
Cheers, Dennis!