"que l'on ait eue" should also be correct!
Aggreed. "que l'on ait eue" or "qu'on ait eue" should be considered correct. We pretty much never use the "nous" in everyday speech.
Do we only need the "l" to separate the que and the on?
Yep. It's either "qu'on ait eue" or "que l'on ait eue" (the latter is a bit more formal), and there is no third option.
' eue ' ?
That is the female singular past participle of "avoir."
Since the direct object (la voiture) appears before the auxiliary very (ayons) the gender and number of the participle must agree with the direct object (fem. singular).
Auto est un synonyme de voiture comme tout un chacun sait (sauf Duolingo)
I must agree with Sunnymeader, auto, automobile and voiture are synonyms unless it would have been specified as a train car… which it isn't