I just fixed mine. It was leaking at the lower right hand corner of the windshield glass. I removed the under dash panel and used a 2 gallons pressured water bottle to spray water onto the windshield, mainly near the corner area. I saw damp patch on the felt under the dash, right underneath the glass corner.

I cleaned the corner well and sealed it with silicon sealant. Redid the test and no more damp patch. Put the under dash panel back and keep my finger crossed. Your leak may differ but it is worth a try,

__________________
Not MBZ nor A/C trained professional but a die-hard DIY and green engineer. Use the info at your own peril. Picked up 2 Infractions because of disagreements. NOW reversed.

W124 Keyless remote, PM for details. http://www.peachparts.com/shopforum/mercedes-used-parts-sale-wanted/334620-fs-w124-chasis-keyless-remote-%2450-shipped.html

1 X 2006 CDI
1 x 87 300SDL
1 x 87 300D
1 x 87 300TDT wagon
1 x 83 300D
1 x 84 190D ( 5 sp ) - All R134 converted + keyless entry.