Not sure who can clean it as you'd have to take the lens off to do it properly, and that's near on impossible to do without ruining the lens.
If it were my car i'd take it out of the car, remove the bulbs and either stick it in the dishwasher or leave it to soak overnight in soapy water.
If that works, get a 5mm hole drilled in the lowest part of the light casing to allow it to drain out, but more importantly find where the water is gettin in and seal it up with mastic or silicon.
If it doesn't, find a good used replacement light unit and fit that instead.
Not sure who can clean it as you'd have to take the lens off to do it properly, and that's near on impossible to do without ruining the lens.
If it were my car i'd take it out of the car, remove the bulbs and either stick it in the dishwasher or leave it to soak overnight in soapy water.
If that works, get a 5mm hole drilled in the lowest part of the light casing to allow it to drain out, but more importantly find where the water is gettin in and seal it up with mastic or silicon.
If it doesn't, find a good used replacement light unit and fit that instead.