"friend was having water problems with the rear window on his newly purchased 1980 240d. "
For your friend....tech bulletins.....


67_009 APR 82 Rear Window Sealing Frame - Modified

97_012 APR 84 Body - Mercedes Body Seam Sealer Precaution

97_015 SEP 86 Body - Applying PVC Undercoating and Seam Sealer

72_025A MAR 83 Window Frame - Reveal Moulding Replacement

from: http://alldata.com/TSB/35/80350202.html