The official one is the Grundig MCD36 but as far as I can tell, any Grundig changer will work.

Grundig CD changers were also available in some Fiats.
Although the official CD changer was 6 disc you can get 8 or 10 disc versions however, accessing any
disc above 6 was a bit of a pain as you'd have to cycle through all of the songs on disc 6 to get to 7 etc.
Apart from needing the CD changer itself you will need the Grundig cable. Circular on 1 end and an 8 pin
blue square housing on the other end. They tend to be too long but you can hide the remainder of the
cable in the passenger footwell underneather the polystyrene block that covers the battery.
Smart had a special under seat mounting system but it's really not necessary.
A few strips of sticky backed Velcro (hook side) stuck to the underneath of the CD
changer unit allowed it to securely stick to the carpet under the passenger seat.
You must have a 450 or Roadster with a Grundig stereo. It won't work with the 451 stereos, forfour
stereos or the 1 DIN Becker Navigation & Sound stereo for the 450 and Roadster. Nor will it work on
3rd party or after market stereos unless they happen to be Grundig as well.