Hi all I have an issue with water in the tail light casings (both). I can't seee anywhere the water is getting in. Not sure what to do

They look to be after market rear light clusters so that could be your problem  2003 BMW M5 - 5 Litres of V8 Optimax Slurping Goodness in Carbon Black

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pop both light clusters out including the bulbs and if possible use a hairdryer in the light holder's to dry them out if you have someone spare get them to hold a piece of paper around the front of the lense and rubbers as the hairdryer blows inside it should indicate where a crack or if the rubber is apart and letting the moisture in