Do yourself a big favor. Before you do ANY more diagnostic work go down to your local parts vendor and get a full set of fuses for your car. Mercedes Fuses are known for corrosion issues and while they might "look" good they might not be providing a good connection. Also don't do the glass fuses. They never work quite right and have a tendancy to blow frequently.

I had a number of electrical issues in my 300SD that completely went away with new fuses. They are cheap and will save you a ton of headaches.