Given the year, my guess is that you have the same system I do in my 1996 S500. The interior rear view mirror has a red and green light. Attached to the very bottom of the mirror is the IR receiver. It looks like a dark/black protrusion.
This configuration works so well they don't use it any more.
On mine, even with fresh batteries, the operating range of the remotes to the receiver is very short. The remote IR window (red plastic) must be pointed directly at the rear view mirror. I think the max I've ever achieved on a brightly lit day is about 5 feet. Its a bit better at night, but not much.
Get close to the mirror with the remote and try it. Do either/both of the red/green lights on the mirror blink? Give us the sequence/pattern of the blinking lights and we may be able to help.
Hopefully, the remote you have is the right one for the car. If not, you'll have to order a new one from MB. (ouch!)
Contrary to what many sellers on eBay would claim, you cannot take any MB IR remote that "looks" right and program it to the car... its got to be one of the originals, or a MB provided replacement.