hey there kelleymiller, welcome to the forum!
You just never know what might play or not play in car dvd players. Mine for instance will only play dvd-r disks. A friend of mines’ player will play dvd+r or dvd-rom, but not dvd-r. Players can be very fickle. You might want to try burning different media, at a reasonable speed (not too fast), but most importantly, use GOOD media. Such as TY or Verbatim.
Hope this helps…let us know how it goes!