I wouldn't think a add-on CD Changer in the trunk could void your warrenty.
Changing the head unit or adding an amp could keep them from replacing speakers etc.
The first thing I did with my 2001 Elantra was replace the head unit with a MP3 player, change all the speakers (The factory ones are crap). I added 2 amps and 2-12" behind the rear seat.
When voiding the warrenty I have heard they must prove that something you changed or did caused it to fail. There are web pages on these subjects..... <img src=/images/forums/snitz/smile.gif width=15 height=15 border=0>
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