CLK Audio System improvement....for free!
Here's a simple and free way to really improve the audio quality on the 2006 CLK. The problem of excess midrange (harsh, shrill sound) is caused by the 3" midrange speaker in the center of the dashboard! The solution: disconnect it! Get a "tongue depressor" or popsicle stick, and pry up the grill that covers the center dash speaker. There are two pair of wires that connect to the speaker; just unplug both of them, and re-seat the grill.
The improvement is dramatic. You can actually hear highs from the tweeters. Setting the bass, treble, etc. is subjective, but I set mine as follows: bass to "0"; treble to "+3"; system in "Standard" mode.
A huge improvement, for zero cost!