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You may want to wait for someone with a C6 manual to chime in, but if you're in a hurry...
From what I've been told, for the front speakers the lighter colored wire is the +, and for the rear speakers the darker colored wire is the +.
You can click here for the C5 speaker wire schematics. Compare them to what you see in your car, if the speakers in the same locations have the same color wires they should have the same polarity.
get a 1.5v (C or D cell) battery and conect it to the original speaker - the cone will push out when the + is on the correct terminal
you can use paper clips for this test if it is easier (i do)
hope this helps...