I'll try to explain:
Usually PBAP uses UTF-8 for character coding. This can represent all characters that ever existed in the world, including Egypt hieroglyphs and Sumerian cuneiform. I'm not sure with Klingonian
Cars usually cannot display all existing characters. I guess no car will display Klingonian or Egypt hieroglyphes, some will show Asian symbols or Greek or Cyrillic characters, but some even cannot display Latin characters with diacritics, like č. Usually the car then displays a simple space or some fancy symbol (which I'd consider a bug in the car software).
Some cars cannot handle UTF-8 (Volvo as far as I remember), that's why it is possible to select the character encoding in my app. This is worth a try if the car shows 2 completely different characters for a single "special" character, e.g. 'Ä ' for č.
If your car cannot display the characters you have in your data, it cannot display them in whatever coding
you use. To prevent the ugly gaps, you can use the transliteration plugin to replace the missing characters with similar looking charaters, which the car is able to display, like c for č.