I've got a CD changer in the boot with the Stream head unit. The Stream is badged MP3 - does this mean the autochanger will also decode MP3 discs or does the MP3 designation only apply to the CD player in the head unit?

I miss the USB/MP3 port I had on my last car - is there an easy way to add a USB option to the Stream system?

Mp3 is on the head unit only and not the changer

I miss the USB/MP3 port I had on my last car - is there an easy way to add a USB option to the Stream system?

not USB, but you can add a standard phono aux-in fairly straight-forwardly using a Connects2 adapter, which hi-jacks the CD changer port in the back of the headunit.

Quite a few threads on that in the ICE section (and probably in here too, if you search for "Connects2").