Water may be entering the back of the vehicle though the leaking tail light. Inspect the spare tire well for water, that may be the source of water that is found in the back seat floor.
I recommend checking the AC drain in the front passengers’ side foot area. If the drain is clogged, water will leak under the carpet and find its way into the back seat floor area. … If the water is on top of the trunk mat, then it is possible your back glass seal is leaking, or the trunk lid gasket is leaking.
What is the trunk lid gasket?
I have an Impreza WAGON.
The water is now on both sides and much more after lots of rain.
It’s only the floors and not the seats so I’m inclined to not blame the windows.
I guess you will need to take this vehicle of unknown age to a body shop and hope they can find where the water is leaking into the vehicle . I assume you have not checked any of the drains .
2006 Sub Impreza outback sport wagon.
I have not checked drains as I don’t know where they are.
I will google it and see what comes up.