Car Stereos: 1999 Volks Wolfsberg edition, stereo problem, antenna wire, radio works

Question
My daughter's car stereo is not working.  AM of radio works, but not FM.  Hooked to a CD changer in the trunk and that is no longer working.  We have checked through the manual and cannot find any help as to what could be the trouble.  Thinking of replacing it, but know that is tricky with the security codes.  Can you direct me to what stereo I need to purchase and how to do that?  Also, you mentioned an external amp in an answer to someone else's question.  Could that be the source of our problem?

Answer
Kelli, Since the AM still works, then the power end of system is working fine.  I would guess that perhaps the antenna wire has come loose, or has gotten broke internally.  I would have the wire and connection checked to make sure they are working properly, and that should fix the problem.  Hope this helps, Scott