My '94 S500, the DTCs will 'erase' if you pull the module out of the F23 box or reset them with X11/21s1 (pb/LED).
Never tried David's 'favorite', disconnecting the battery. If pulling the module erases all codes, then so will disconnecting the battery for my car. And it looks like it will for most pre '96 models. I know that some C class cars had OBDII on board before '96.
Here's an OBDII link: http://www.obdii.com/obdii.html
Always learn something with your posts. Tnx for OBDII info. My car is not OBDII compliant, thank God!