Not an expert, but a couple things come to mind.
Are you trying to use MP3 format in your car unit? Or did you convert them to 44.1Khz Wav files first?
I burn quite a few Music CDRs every week (Zep bootleg freak) and they play in everything. I only burn Music at 4x though....not tried faster with the NEC, but the only error I've ever had was a faster burn with my last burner. And some feel that the slower burns make for better burn pits. True or not? Who knows. I also use Nero 6.3....and the default firmware for now.
You do know that MP3's are a "lossy" format (compression format, like JPG's...lose's data) and have a high frequency cut-off around 17-18Khz. Not very good for a true music lover. (but handy for portables where the trade off is ok to some)
All of my Boots are in one of two "lossless" compression formats.. .shn's and .flacs. I convert them to wav's (another lossless format) and they run on everything I've tried, even 10yr old boomboxes.
I do burn my Data files at the fastest speeds I can. It's only data!