Yeah, “faisez” would be the logical form for the present indicative/imperative accompanying “vous,” to someone who didn’t know the verb was irregular. I was confused when I saw this phrase for the first time yesterday (posted by Ramira, who I know is a native French speaker).
I was only copying it directly from this thread for the purpose of a funny to me response to the idiocy of a certain pit thread although to avoid confusion my mother tongue is not the French, it is my school language (and first work language) and third language of the five I speak.
I would think if you were to make that kind of error you would use fairez.
The closest thing is faisiez, which is imparfait and wouldn’t be correct for the usage on the bumper sticker.
You’d get beat up for having some sissy foreign language sticker on your car. Worse if your car isn’t made in the USA.
Sorry, my mistake.
Maybe - depends on how far into French 101 the speaker has gotten. 
Faisez seems the more likely error to me since regular verbs like taire and plaire are conjugated as taisez and plaisez.
The rules change on odd numbered days anyway.
possibly referencing this?
Probably a truncation of ne fais pas une gaffe. Which is don’t make a mistake. My understanding of spoken french is that it is common to leave out the ne which is most of the way there. Kind of like how “I could care less” came to mean “I couldn’t care less.”
Despite the appearance, “faire gaffe” (to pay attention) is unrelated to “faire une gaffe” (to make a mistake). It comes from an old French word meaning “sentry”.