Water leak into trunk

Hunting water leaks is a pain in the tailbone job to do. I'm assuming yours has the jack in the floor of the trunk, under the carpet.
The spare wheel well is the lowest point in the trunk so the water will collect there regardless of where the leak is. Start with the simplest tools, a flash light and the palm of your hand.
How much water is collecting and how fast does it fill up? Rain water coming up through the floor is unlikely unless the floor is damaged.
First take everything out of the trunk that wasn't in there when it came out of the factory. Take a really good look at the carpet and trunk side liners. Use the palm of your hand and feel the floor carpet for wetness. Your feeling to see if it is wet in a particular area. Using the flashlight look for darker patches as carpets often darken in color when wet. Move to the rear of the trunk and feel the liner on the back of the rear seats. Next run your hand over the side panels where the wheel arches are. This will give you clues as to where the leak is possibly coming from.
If the whole trunk carpet and liners are bone dry the leak is from under the car.
All the doors, the trunk lid and the sun roof are fitted with rubber seals to stop rain and road splashes getting inside the vehicle. Over time these perish and start to leak. Run your hand around the underside of the rubber seal the trunk lid sits on when closed. This is the most obvious place it will leak from. You can do this on each door as well because water can travel a large distance before it finds somewhere to create a puddle. A Sunroof can also leak but they tend to cause puddles in the foot well.
If you get a few dry days you can test for leaks with a hose pipe. Keep the trunk closed and make sure the inside of it is totally dry. Remove any spray nozzle from the hose so it flows at regular pressure. Carefully aim the water so it only flows over the trunk lid. Let it flow for a minute or two trying not to soak the doors at the same time. Turn off the water and leave the car alone for about 10 minutes. Wipe off any excess water on the trunk lid, open it and see if its now wet inside the trunk. Yes? the trunk seal is leaking. You can use this method for the doors and sun roof as well. Door and trunk seals are usually just a 'Push on/Pull off' fit so you can easily change them yourself.

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

Can't fix anything till you find out where the leak is

Get inside & have someone run the hose on it

Remove whatever trim you have to,to see the water
entering

The rubber water shield (weatherstripping) around the trunk opening must be leaking. Feel around for moisture along and under the rubber strip. Maybe a tear is letting water in. Maybe the rubber came loose from where it is glued down. If you find a possible source of the leak, use rubber cement (or weatherstripping glue from a parts store) to seal it back up.
Good luck.

There is no drain plug. For the leaks be certain to investigate: sunroof drains, taillight seals, and the trunk seal. Many trunk seals actually channel water into the trunk. New designs are available on many GMs.

The two main causes are: 1) the rubber seal on the body where the trunk lid closes. check for tears or where it is not seated properly
2) The rear window is leaking around the seal
Of course if you have an antenna or the like which has been mounted on the rear fenders that seal could be the problem.

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.

same problem happened to my 2005 bonneville. inside the trunk and behind the carpet covering the wheel wells there are two water drain tubes. these tubes run down the inside of the "c" pillar and drain the water from the sunroof. they make a 90 degree bend at the bottom back of the wheel well(inside the trunk) and fit inside a round rubber tube/grommet at both of the rear wheel wells that lets the water drain outside.either the plastic 90 degree tube has come out of the rubber tube/grommet or the drain tube could be plugged up with debris. also make sure the two round rubber tube/grommets are installed correctly and seal the hole going through the trunk to the outside of each wheel well. this is of course only for those bonnevilles that have a factory sun/moon roof.

CHECK TO MAKE SURE THE WEATHER SEAL AROUND THE TRUNK LID IS NOT DAMAGED.IF YOU CAN GET SOMEONE IN THE TRUNK WITH A FLASHLIGHT TO SEE IF WATER IS COMING IN AROUND THE SEAL WHILE YOU USE A HOSE TO SOAK THE TRUNK LID GAPS

Sounds to me like your issue is with condensation from your air conditioner. The condensation isn't properly draining, probably from a damaged or clogged drain hose.