There are three bumper stickers that I see fairly regularly, all three of which I viewed in transit in one day. They all have two things in common: they display a postmodern mindset, and they use terrible reasoning. (It can be successfully argued that these two things go hand in hand) Almost always they are accompanied by some other intellectually bankrupt and poorly reasoned bumper stickers, but these three are by far the most common.
Bumper sticker number 1:
This is by far the most overtly postmodern of the bumper stickers I have seen.
To question reality implies that one can also reject reality. If one cannot reject reality in any situation they choose, then one cannot ever reject reality. That is, one cannot arbitrarily reject reality. It truly is an all or nothing belief.
If we are to question reality, how does the reader of the sticker know that they are, in fact, reading a bumper sticker? How do they even know that that car exists? The madness ends when we must appeal to reality in order to draw the "question reality" imperative. Therefore, it refutes itself.
Bumper sticker number 2:
Including the symbols of all the major religions comprising the letters in the word, this was paired with a sticker from a Yoga gym as well as a "Free Tibet" sticker. What is the connection? The answer is: if it walks like an Eastern religion and talks like an Eastern religion, then it is probably an Eastern religion. The sticker then raises the question, what does it mean to coexist?
According to http://www.dictionary.com/:
–verb (used without object)
1. to exist together or at the same time.
2. to exist separately or independently but peaceably, often while remaining rivals or adversaries: Although their ideologies differ greatly, the two great powers must coexist.
I doubt very highly that the driver of this car would go out of their way to purchase a sticker in order make a statement that we should all "exist together simultaneously while often remaining adversaries." (combination of definitions mine) This would (correctly) imply that all are indeed coexisting as long as we are not dead or trying to kill each other. No, what this vehicle was attempting to get across was that we are all equal and that no one path is correct. Facing this definition, the creator of that sticker might as well replace the symbol of Christianity as well as the symbols for Islam and to some degree, Judaism with something else. Their intrinsic natures require that only one can be the true path. The presence of these renders the whole idea null and void.
Bumper sticker number 3:
Showcasing the apparently admirable postmodern trend of not submitting to authority, the decal was paired with a "Darwin fish". This is a significant pairing.
I will explain. The purpose of the Darwin fish with legs is to announce to the world that we are all accidents and, by logical default, life is meaningless. Therefore, this concept of questioning authority has no real intention to remain for an answer, as answers imply meaning. It is just merely empty questioning. And people say that Christians are closed minded! What the sticker is actually communicating is "Reject Authority".
If we are giant cosmic accidents, there is no natural need for authority. In that respect, adherents to the sticker (no pun intended) are being logically consistent, but they are still dangerously wrong. Rejecting authority logically progresses to supporting anarchy, which almost always progresses to a totalitarian regime such as communism, or even socialism which is soft communism (on that, see http://www.marxmail.org/faq/socialism_and_communism.htm).
This sticker also refutes itself, in that "question authority" is an authoritative imperative. One must, if taking it seriously, question the statement itself. Therefore it is without meaning.
This analysis only begins to scratch the surface of the inconsistencies of these stickers and the cultural milieu surrounding them. There are much more thorough resources out there which delve into these postmodern inconsistencies; I recommend they be utilized.