According to my dictionary*, “prius” is an adverb (meaning “previously, before”), and so it does not have a plural.
“Prius” as the name of the car is a noun, and so, regardless of whether it is taken from the adverb or not, it will have a plural. But how can we decide whether it is a second declension noun, and thus take the nominative plural “prii” (as Rat suggests), or a fourth declension noun, and thus take the nominative plural “priūs”?
Can anyone help?
(*D. A. Kidd, 1959, Collins Latin Gem Dictionary, Collins, London and Glasgow.)