auto accessory vs add-on

auto accessory and add-on both are nouns.

Word	Noun	Adjective	Verb	Adverb
auto accessory	Yes	No	No	No
add-on	Yes	No	No	No

As nouns, add-on is a hypernym of auto accessory; that is, add-on is a word with a broader meaning than auto accessory:

Other hypernyms of auto accessory include accessory, appurtenance, supplement.

auto accessory (noun)	add-on (noun)
an accessory for an automobile	a component that is added to something to improve it
	a supplementary component that improves capability
	a software module that extends or enhances the capabilities of an existing application