I also have an 81 300SD which had a VERY wet trunk when I bought it. Now keep in mind, this is my first older Mercedes deisel and I knew nothing of the chronic ailments of this car let alone the existence of this forum! So, first I installed a new real window seal, it helped a little. After that, I climbed into the trunk with a flashlight and had the wife hose down various parts of the trunk area. Low and behold, very leaky tail light seals! So, I removed both lights, discarded all old seals and cleaned all surfaces. Then came the black silicone sealant! Keep in mind, this car is my driver, I wouldn't do this to a nice car. Well the sealant worked wonders, my trunk is now dry as can be!!!!