Haasman,

I never met some fiber optics in a W 124 ! No, when you take the bulbs out of the cluster with their sockets, that will leave you with an opening large enough to get in with a cottonwool covered baton as one uses to clean out his ears or better, the longer ones used to clean the heads of a cassete-player or the lens of a CD-player. In fact I used the same cleaning fluid : it's alcohol based, volatile and doesn't attack the white plastic of the light "channels".
Archaic as a method but very effectif !
Next time you pull out that cluster : try it out.
I only hope your resistor to dim the lights is still working ? You'll need it !

Danny