For your reading "pleasure", here are two entirely vapid essays on multiculturalism. Multiculturalism, citizenship and national identity and The many faces of multiculturalism. Neither says much of anything at all, much less anything interesting.
As I've noted before, "relativism" by itself is vacuous. Everything is relative, in some sense, to something else. Just saying that culture is relative to the people who comprise it is not by itself a particularly interesting observation. Which properties of the individuals are relevant to culture? To the extent that culture is anything at all, as some sort of abstraction of those properties, what sort of abstraction is it? What specifically is relative to what? What specifically is invariant with respect to what?
There are no principled, philosophical answers to the above questions. There are no objective normative truths what properties should comprise a culture, what abstraction culture should represent, what should be relative or invariant to what. We must ask, rather, scientific questions: How do people actually construct "culture"? How do people actually want to construct culture? How can an individual make rational decisions about these questions to maximize her self-interest?
We can answer the first question through the sciences of sociology and anthropology in the usual scientific manner: Construct theoretical frameworks consisting of falsifiable hypotheses, test them against reality and, accept them if they're supported by the evidence. Especially with something as complex as culture, there will be many perspectives, many theories, which can fulfill these scientific criteria and give us useful insights; until Hari Seldon appears on the scene, we won't have any sort of grand unified theory of sociology. This sort of approach is "postmodern" in the sense that it privileges a priori neither an overarching metaphysical notion of "culture" nor any particular scientific perspective. It is still scientific, though, in that only those perspectives with evidentiary meaning and support have any currency—thus excluding the trivial vacuity of the linked articles on methodological, not metaphysical, grounds.
I'll leave most of the sociology to professional sociologists, but one meta-theoretical perspective is glaringly obvious: Culture is, like personality, normatively self-descriptive. Just as "who I myself should be" is a matter of absolute personal privilege, "what culture X should be" is a matter of absolute privilege of the members of that culture.
On the other hand, actuality is a matter of real scientific truth: "Who I myself actually am" (as opposed to who I should be) is a matter of real truth and can be empirically determined. It should be noted that an abstract quantity, "Who I am" has different answers (i.e. is relative to) depending on how the question is specifically constructed. One can look philosophically at this sort of scientific "relativism" either as perspective relativism or as property pluralism: One can get scientifically true views from different perspectives, or one can scientifically determine different properties. The meaning of the perspective or property relative to the way the scientific question is framed.
In precisely the same way, what a culture is is a matter of real, empirically determinable truth, and relative to the sort of scientific question asked.
Since the normative character of an individual or a culture is self-defined, one must, fundamentally, ask questions of the individual himself or the members of the culture themselves. One can collect evidence from outside the individual and culture, but the evidence must causally connect with the individual or members to be about that individual or culture. You cannot tell much of anything about me, for instance, by examining the character of my wife.
On the other hand, since all the members comprise a culture, no subset of a culture can, by itself, authoritatively define that culture. One cannot, for instance, determine what Islam is by repeating what one Imam says about Islam: You can determine only what Islam means to that particular person. Since many members comprise a culture, only statistical abstractions will have any meaning.
I'll leave the rest of the general sociology to the professionals, and repeat my two philosophical exhortations: First, don't get hung up on a priori metaphysics, just ask interesting questions about reality. Second, make sure you're asking scientifically meaningful questions, i.e. questions that are empirically falsifiable. It has been our exclusive experience that when you ask enough scientifically meaningful questions, coherent theoretical frameworks will emerge from the answers.
 Constructing "real" as the union of the subjective (minded) and the objective (non-minded).