I have done that as well...
You need to exchange your front courtesy light for a newer one (with the dimming control). It is a feature that is build in the courtesy light itself.
I exchanged my front (1993) courtesy light for a dimming one once, but this didn't work because my rear courtesy light would not go out anymore after the car was locked. When I turned my ignition key (after getting in) the bulb in the rear courtesy light was buzzing whilst it dimmed out...My rear courtesy light did not communicate to well with the dimming front courtesy light!
I don't know whether this rear courtesy light problem was caused by my after market alarm that was hooked up the door contacts or whether there are some changes in the wire harness of the pre and after 1995 models.
After I exchanged my complete interior and some of the wire harness into that of a 1996 C36 AMG, the front and rear courtesy lights dim perfectly.
David: start by getting a dimmed courtesy light out of the C class of one of your (girl) friends (since you said once that they have newer C classes) and see what your rear courtesy light does in combination with the front courtesy light.
If this works out fine, you can exchange the complete front courtesy light to have that simple (not necessary) feature.
with a minor 600+k