The unit fitted in mine is what's referred to as the "Hi-Line" and uses the Alpine R990 head unit and includes the 6 disc CD auto-changer under the driver's seat and the power amplifier and sub driver under the front passenger's seat.
There are two other installations in the D2, depending on version, the "Mid-Line" which has a single CD slot and I don't think it has the power amp either. Then there's the H-K (Harman Kardon) which is the top of the range and includes the built-in Sat-nav.
As far as I'm concerned, it works well, if you're an audiophile or you want loads of bling then it won't suit you. The Hi-Line has 12 speakers, a mid and a tweeter in each door, a tweeter in each "A" post and a pair of "sort of bass units" in the rear tailgate.
The aerials are etched into the glass of the two rear side windows, the AM aerial on the offside and the FM aerial in the nearside glass. Each is fed through a pre-amplifier.
Without modification, it's not possible to feed the output from an external source such as an mp3 player or whatever.
As I said, unless you're an audiophile it's OK, but remember, you're still listening to music in a steel box and not an audio theatre.
Replacement head units do occasionally come up for sale on the bay.
Some members have replaced the head unit with more modern equipment to enable reception of DAB and also connection of external sources. I've achieved that by using the original fittings and a DAB adapter unit.