Hello Uga487 and Welcome!
To answer your first question, an MP3 is an MP3 regardless if it is on a Mac or PC. It's a universal extension.
Now on to your Car's CD player. Make sure you go into iTunes, Then go to Preferences and select the Advanced Tab --> Click on the Burning Sub-Tab --> Make sure MP3 CD is selected. After that's done, make a playlist by going to File --> New Playlist. Put some tunes in it and then click the burn Disk on the bottom of iTunes. If you did these steps already, then I apologize for being redundant.
I would suggest to try burning another disk and see what happens. Sometimes, MP3 disks can be picky. They don't like to play on all MP3 enabled Car Stereos. If you are still having problems, try to find another CD brand.
I use Memorex disks which seem to be fine.
Let us know how it goes.
Cheerio
Searay