I am trying to be positive and not giving you a bad time: Just some hints as to what/where you should be looking at with a car that is 15 years old. Yes, please use the search box for sticky key fix (your's may already be fixed, but you should be aware of this problem), also search for water in trunk. That pertains to sealing your wing mounts.

Does your siren sound when after you push down on the panic button on the key FOB for a couple seconds? Yes, you are good, No then you need to remove it or replace it. It is also under the cowl.
Changed all the fluids? So you have had the brake system flushed with all new fluid, and what about sucking out the differential oil and have that replaced. It is probably 15 years old. As little as you use your car, perhaps the windshield washer fluid has green stuff growing in it. Do you use those washers often and has that fluid been sucked out and replaced. I'm guessing it may be 15 years old. And what about the transmission. Has it ever been serviced; pan dropped, new filter and correct fluid exchanged? What about steering fluid? Sucked out and replaced with the correct Mercedes fluid. DO NOT TRUST A GARAGE TO USE THE CORRECT FLUID REPLACEMENT UNLESS YOU ARE IN A MERCEDES SHOP.
The RCM will give you problems. It would be beneficial to have a spare available as it is a simple swap when it starts to give trouble.
Did you put a new belt on after repairing the alternator and water pump?
Hope that is helpful.