Helped somebody refurbish a light the other day we filled the assembly with water to check for leaks (then oven to dehydrate). You could pressurize and use soap spray.

Hint: you said crumbly foam that might be key to keep water out. Open cell foam so only air no water gets through.

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2011 E70 • N55 (me)
2012 E70 • N63 (wife)