Have Mercedes Questions? Ask a Mechanic for Answers ASAP
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If the seal around the windshield is leaking this can allow water to enter your vehicle and cause this here.I have seen some of these leak before and had to either re seal or replace the windshield as sometimes a reseal is not possible.
The other area is the ac drain if the water appears after the ac has been used.The drain usually drains out of the bottom of the vehicle above the transmission.Look for a drain hole or tube under the vehicle and use a mechanics wire or similar to push into the drain slightly to see if you can free up and remove any debris to correct this here.In allot of cases it can just be gummed up with debris and need cleaned out this way to correct this here.
You may have to peel back the carpet in the front seat area to inspect the drain for any disconnect issues needing corrected.
If all of these are o.k. this may be a roof seal leaking allowing water to run down the A pillar post into the foot well area.I would have the roof seals checked for any wear or damaged or being collapsed needing replaced possibly.
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