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Well without getting a diagnostic computer that can check to see if the fiber optic ring is down, correct diagnosis is a little harder. I would expose the amplifier behind the right trunk panel and pull the fiber optic connector out and then turn the radio on. Look for the blinking red light within the first 30 seconds or so. If you see it, the rest of the fiber optic ring is probably working and it just needs a new amplifier. New amplifiers are very expensive but you can get one on ebay fairly cheap. http://www.ebay.com/itm/OEM-Mercedes-05-09-W203-W209-W169-C280-C230-C55-CLK350-CLK550-HK-Amplifier-Amp-/381437342565?hash=item58cf6ff365:g:pfQAAOxydB1Sg-h6&vxp=mtr
Please let me know if you have any more questions before you rate me, thanks!
If you have a convertible CLK, the amplifier may be on the other side.