The upper left-hand corner of my windshield (89 300E) drips in heavy rain. Short of re-bedding the windshield, is there a source (besides the immediate corner which I sealed) that may be feeding this leak?

Thanks in advance!

 

There is no known problems of such a condition.
The seal is bad or if the ws was replaced at some time, not done properly. Remove all the surrounding metal so you have exposed all bare glass and inspect/reseal.