Alpine-USA once sold the CHM-S601 a 6 CD (M-Bus) Changer but has discontinued it. Used Alpine CHM-S601 changers are offered on ebay.com for $50 to $100; they get mounted, pluged in (and hopefully play). These old players have no MP-3 capability.
Alpine-USA now offers CHA-S634, a modern, 6 CD (AI Net) Changer that includes MP-3 capability and it should work in this car with an Alpine KCA130B AI Net to M-Bus Adapter, a short cable with differing plugs to connect the car?s (old) M-Bus harness to the (new) AI Net Player.
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for instance, discounts this changer to $240 and the adapter to $15.
It?s not clear to me whether this changer can be controlled by the radio or not, so that ought to be investigated if you want to add it.