If you look up at the sunroof from the front seat, do you see calcium marks around the glass? If so, the source of the leak is the rubber seal around the glass. Mine was so bad, the metal portion of the sunroof was rusted through. I purchased another from Strandberg in WS.
Normally, you can buy another rubber seal and contact a windshield replacement company to get some glue to install the new one. Patching the old one with silicon does not work, it expands/contracts at a different rate than the glass and will not form a tight bond.

If the rubber seal is OK, then check the 2 drain holes at the front corners for blockages.

Klaus
The brickboard is not responding to automatic emails, so I may not respond back.
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Please answer your responses. We need to know if the advice is good or bad. The 164 has a new home, all I am left with are 2 turbos :)