Ive did this a few weeks ago. Its not hard as such, but it was only when the Plastic started to crack I accepted you need to go in from behind!. You need to remove the binacle entirely (having disconnected the various cables and wires - make notes if your not familiar with what goes where), (with the W124 you dont need to remove the steering wheel) then take the clocks out and then squeeze the dial rings through the back of the binacle and then pull/press/click them into place. The rings that I bought from Ebay are chrome covered ABS plastic so they bend easily enough to go in. I previoulsy fitted dial rings to my Mitsi FTO which were solid metal, but with the FTO the clear plastic front of tyhe binnacle comes off.
Just thought, which Merc do you have?