I suspect the rear deck speaker on my C-class is blown. crackle/distortion coming singularly from that speaker.
Is there a way I can test or a stereo shop can test to make sure the speaker is blown? it was a pain extracting the deck speaker and I would prefer not pulling the speaker on the other side to do a test

Also what happens that a speaker is blown, what goes wrong with it? what part destructs?

Speakers can "blow" in two distinct ways. First the voice coil can overheat and bind within the magnet assembly. Second, the paper cone can tear, either through age, or by being exposed to a killer signal that tears the cone from the edge of the speaker spider.

Depending on how bad the damage is, you may have to pull them. Sound checking is usually adequate, either through a test signal (most cleaning CD's or tapes have this built in) or through careful listening to a music track that has broad dynamic range (Queen's, "Night at the Opera" CD?). You're listening for distortion at moderate, not full power, listening levels.