I suggested this before. If its caked on stuff you can literally tear pieces of cotton with high grade lens cleaner on it and shove it in there... use a tooth pick and break off the too to give the cotton aomthing to hold on to. Then slowly spin the cotton around. When you think its good just pick it out with your fingers. Usually works for cleaning my wicked llasers executive which believe me... its a pain to clean.
You can also try compressed air but I'd be afraid of sort hitting and scratching the lenses.
That's my suggestion. Hope you get it nice and clean!