MP4 songs from you tube will not convert to mp3 songs all the way?

I have a stereo system in my car that I can hook a flash drive up to; however, I have been having trouble converting the songs from you tube to get the entire songs to play. I use a you tube downloader and save the songs directly to the flash drive; however, the songs are in mp4 format or something, and my car stereo is not supported through the flash drive. So, I converted the mp4 songs to mp3, and they still won't convert but half the song when I play my flash drive on the car stereo. I've been using 4media MP4 to MP3 converter 6. Now, I can convert it to windows media player and that has played on my flash drive in the past, but I don't want to download the songs onto my new computer. I just want to save the songs directly to the flash drive without having only half the song converted. These are for you tube conversions. I've used a website called zamzar.com, but you have to wait for an e-mail with the converted you tube song, and that takes longer. These are songs that I've done, and othe people I know, but only half the song plays when I put it on my flash drive to my stereo. How can I convert songs from you tube in mp3 format or WMP format directly to my flash drive without downloading it onto my computer? Any information will be helpful. Sorrry if I'm not making myself clear, just want to be able to hear an entire song on my flash drive, without opening it up on my computer.