[BigBen]

The Harmon Kardon system uses the same head unit(s), it's just the Amp and speakers that are different.

I think what you're asking is whether or not the wide-screen BMW head unit will fit, which it will; but you will only get the same 4:3 square display size in the centre of the screen as the original hi-line unless you change the control unit in the boot too.

If you want an aftermarket double-din that will fit then there are a few that will work with the existing changer and have sat nav (it's an Alpine) but most people just buy a cheap chinese unit withor without a DVD player.

With one of these then you will lose the CD-changer but will have the ability to us music stored on SD cards or USB sticks.

Also, in this case, it's easier if you do not have harmon/kardon - because then all you need is a radio harness adapter to go from the 17-pin BMW radio plug to a standard DIN plug and you're good to go.

Edit - you'll also need an aerial extension cable.