Ok, Your 05 CLK will have the Phone Prewire (it was an option though). But 2005/2006 was the age where when you had that, it also popped up with a Bluetooth device caled UHI-SNI.... (or very similar), which was for pairing a privacy headset when using a phone connected to the phone prewire.
If you look in your armrest you will find the UHI connector (i.e the phone prewire connector). For bluetooth you must connect a bluetooth adapter to that, and pair that with your phone. If you have the Mercedes HFP adapter, press the bottom of the "top", for 5 seconds until the blue light starts flashing faster, and then pair your phone - the code is 62872
You may find some other cradle plugged in there (for a specific phone), on the top left of the "top" there is a small button - press that and at the same time slide the cradle backwards and it will detach allowing you to then connect a bluetooth adapter.
Richard