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 ...

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.

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. 

 

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.

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

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