If you have a Vios ...

[JAS21]

Suggest that you check the water level in your rear lights .... today I saw two with water sloshing about and have noticed the same previously ... don't think that they were designed to be  water cooled ...

[ferguston]

Also visually  check the operation of the rear lights.

The contacts on the steering column "stalk control " can "burn out" causing the rear lights to fail.

I don't know if Toyota solved the problem on the later Vios models.

[Foozool]

You need to remove the light assembly and seal the possible gap between body of light and exterior piece (red glass) by clear silicone. 

I guess you need to remove 3 or 4 nuts to get the light out. 

I have done this for my DIL car. It is an older Vios model. 

 

[StevieAus]

My wife has a Vios Exclusive just a year old have just checked as she was washing it earlier( thought we must be expecting a visit by royalty) seems to be fine on both counts plus all the rain we have had recently would be a good test.

[Jedsada3]

If there is water in it just drill a small hole at the deepest point of the cover and water will run out...

[StevieAus]

6 hours ago, Jedsada3 said:

If there is water in it just drill a small hole at the deepest point of the cover and water will run out...

Fortunately we don't have a problem but with all due respect to your advice I would have thought the solution was to stop the water entering rather than creating an exit unless your response is to drain then stop it entering