[gtoformula]

Hi - this is Rick from the GTOANT. I had this problem with my Catalina. I crawled in the trunk (it's huge) with a flashlight while my daughter ran the hose. It took quite a while before I found a water trace. If you get water after washing the car it should be easier than was mine.
The simplest area would be around the trunk opening perimeter weatherstrip. This can also be checked like door opening weather strips. Take a piece of paper (about 4" x 1") and close the trunk so that the paper is pinched by the trunklid/weatherstrip pressure and still sticks out far enough to grap an edge. Pull on the paper. Move it around the perimeter of the seal and find where it pulls out without much resistance. That is the area that will need work.
My trunk leak was due to corrosion of the back glass channel. I had to remove the backglass and rebuilt the channel. It still leaked a bit. Turns out that a pin-hole still existed and was leaking between the filler panel (below the backglass) and the inner trunk structural panel. When I found where it was coming in I had to use use a hole saw on the end of a drill to open an area large enough to access and chaulk. Not fun.
Tip: if you put the trunk key in the lock tumbler you can use pliers or a crescent wrench to turn the latch rod while inside the trunk. That way you always have a back up way to exit the trunk (like if my daughter was pissed at me and wanted to get even).