Not sure about your model but on my 300E I was cleaning the A/C drain hose and the foam fell apart. Steve stated that I can install a new foam tubing (got it from the delear $11) by removing the tunnel carpet. I removed the tunnel carpet and the tube was located next to the gas pedal. I removed the broken foam and installed a new one.
Perhaps your drain foam is also bad or your A/C drain is blocked. On the 300E their are two drain hoses one by the driver side and one by the passenger, again, located behind the tunnel carpeting.