I have a '98 e300 and my third brake light failed twice. The problem both times was burnt contacts (metal strips) on the bottom of the unit. The first time, I just cleaned the bad contact and added a little solder. It worked for three years. The second time, I bought a new light at a Mercedes dealer for $55.00 as I felt it would not take additional solder. I thought it was a good price and then discovered that Peach Parts was selling the same unit for $35.00.

This is a Benz World thread that I found helpful:

Third Brake Light Repair '97 - Mercedes-Benz Forum

It is for a '97 light, but it is the same as on a '98.