Hello and Welcome to justanswer....

There are on these, many possibilites Im afraid - several areas where rain water can get in - but it does sound to be coming in from up under the dash -

Common areas here are the screen seal or body joins where water collects in the plenum chamber and runs in -

What you need to do ideally is remove the plastic panneling under the drivers side dash - take the screws out and remove the panels and then use a good lamp to look up under the dash - up under the screen area etc - you are looking for water / rust marks - which will show when traced back - just where the point of entry is - and then you can sure this issue.

There are other methods - such as doing the same as the above and if you dont see any water marks ( though you should! )... have someone play a hose pipe or water can over the screen area while you look to see where its coming in. Once its found dry the area thoroughly and seal as you need / see fit - Usually silicon sealer etc is used to cure such troubles.

So thats the best way forward on this one now to trace this down/ cure this for you.

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