Forester Ltd 2000: CD player

My Forester Ltd 2000 didn't come with CD player.  I wanted to add a CD
player in but could not find any thing in the accessories at Subaru.com.au
for Forester Ltd 2000.  I checked with the local car stereo dealer and they
say if I added the CD player, I will have to take out the original Clarion
Radio/Cassette player and didn't get a good answer as to why.  Is this
correct? Why? Can I get a CD install to work with the original Clairon
Radio/Cassette player?

Thanks.