Why does my CLK 350 2006 battery completely die after two days of not driving it? I replaced the battery in December 2011 because it has always done this.

You have some electrical issue that is putting a drain on the battery. Seems to me the dealer is refusing to look at the issue. Try taking it to a MB independent garage near you for them to diagnose the issue for you.

Tell them to check for a draw , and alternator out put or change battery .

Could be a trunk light, glove box light or hood light that stays on ... thus draining battery every 2 days. I think if battery is fully charged your car should be able to sit at least a month or longer before the car won't start.

Does your car alarm trip on and off in silent mode killing the battery.