I just wanted to follow up on a post I placed here many months ago...at the time I was having a problem finding the source of a leak that caused my '93 Impreza hatchback's spare tire compartment to fill with water every time it rained.
As it turns out, the silicon sealant at the taillight assemblies was the culprit. It had disintegrated in spots, and water would fill up the taillight housing until it overflowed and subtly trickled behind the plastic trim, under the carpet, and down into the spare tire compartment. It would pool there and everything in that compartment rusted badly.
The vehicle's design seems to lead to this...the two gutters on either side of the roof drain backwards, along the outside of the hatch door's seal, and empty out directly on top of the taillight housings--an almost level surface. The water must find its own way off from there, or pool and eventually find its way inside the light.
I repaired the problem by removing the entire taillight housings (there are 5 screws on each I believe; one is hidden far around the side of the car's body and the other four are easy to find), scraping off the old sealant, and replacing the housing with a generous helping of new sealant--just standard 3M stuff from Home Depot. I also added a lot more of the stuff on top of the joint between light housing and the car's body, so that there would not be any depressions where water could pool and start the process again.
I hope someone finds this information helpful!
