Water leaks in around trunk lid, tail lamps - 1998 Pontiac Bonneville

Unless the trunk lid is warped or out of adjustment away from the trunk lid seal it is the seal .

New trunk seal , that rubber thing around the edge of the opening !

The tail light has a leak in it someplace. it fill up and then drains, check the gaskets carefully and look to see if any drain holes are plugged

You must have a leak, check the seal for the trunk and make sure it is properly installed all of the way around. Also remove both tail lights and make sure there is not damage to the panel behind them. remove the side carpet panels in the trunk and look for signs of water trickling in. I have actually had to remove all of the trim and carpet in the trunk of a car and climb in with a flashlight while someone else sprayed the back of a car at all points and look for a leak. Good luck with it.

Leaking trunk seal or rear window sealer also check the tail lights are mounted properly and that the trunk is closed all the way

It could be coming in from around the trunk seal or tailights or maybe a body seam. What you need to do is get inside of the trunk with a good flashlight and have helper that you trust shut you in there and hose the car down with water. Have them spray around the seam of the trunk lid and around the tail lights and hopefully you will see where it is coming in at.

I think you should be looking for the water leak first ! The trunk rubber seals and the tail lamps are high on the suspected list. The drain hose you are looking at could be the fuel tank filler vent hose , necessary to stop vapor lock in the fuel system . Remove the trunk liner and replace with newspaper and check daily for water tracking daily . good luck

I changed all the bulbs in the tail lights after cleqaning all the connections, then sealed the lens around the trunk with clear silicone sealant. This solved most of the problem. The rain would get in around the lens and short the connections. The emergency brake light indicator and the ABS indicator still go on and off, but I havent checked the sensor yet.

Here is the best way to find out where the water is coming in. Get inside the trunk and have someone you trust close the trunk. make sure you do it when it is daylight or a well lit garage. while you are in the trunk with the lid closed you will be able to see light where there is any gap around the trunk seal and that will be where the water is coming in. some times the sides of the trunk opening gets bent down causing the trunk gasket not to seal properly.

My 1990 Honda Accord does this too.
When I wash it or when it rains. If I open the trunk/deck lid water flows out of the right rear tail light housing into the trunk.

Solution: get some rubber gasket sealant and apply under the lower gasket around the rear window on each side at the corners. Allow for drying time. Close the trunk and try some water if this doesn't work you will likely have to replace the rear window seal.

Best of luck.