2001 C320 Tail light issue

[FastEddy]

I have a 2001 C320 Sport purchased in late 2000 with 24,000 miles.

In the last 2 months I have had to have the dealer replace 5 tail lamps on the drivers side. All different lamps. After the 2nd one went out I had them replace all the lamps with new ones. They still kept blowing. When the 4th one went, with much resistance from the dealer I had them replace the circuit board that the lamps mount to. They said there was a small amount of corrosion in the sockets. 4 days later a lamp blew on the passengers side.

At this point the car is 3 days out of warranty. I immediately took it back to the dealer. They have had it for 5 days now and I when asked whats up, they tell me they are testing and cant find anything wrong. They also say they are trying to get Mercedes to cover any work under warranty.

1- Has any one heard of this being an issue with others?
2- What could be causing this?

Something doesn't seem right to me.
I will be talking to them at some point this week and would like to know a bit more about what they may or may not be telling me. Could there be a major issue that the dealer doesn't want to bear the warranty expense on?

Past warranty work on the car has been, electrical failure of the center console and replacement of a window motor.