MP3, WMA and WAV are all data file formats for storing musical data in regular computer files. None of them will play in a traditional CD player. Unless your car player is specifically designed to play mp3 files on ISO discs, you must feed it only CDA (CD Audio) discs. In Nero, select "File"->"New..."->"Audio CD", then drag the mp3 files into the compilation window. Nero will automatically convert the mp3 files into CDA format and create an audio CD that your car player can play.
You say you are ripping MP3. From what are you ripping it? If you are ripping it from audio CD's in the first place, then just skip the conversion into mp3 and back to CDA format and simply drag the source tracks directly from the source CD into the compilation window in Nero.
You might want to read through the following thread to improve your understanding of the difference between mp3 data files and CDA format audio CDs:
http://www.cdrlabs.com/phpBB/viewtopic.php?t=8025
(You can ignore the parts specifically about CD Text and just concentrate on the portions that discuss the differences between MP3 and CDA)
cfitz