Alero trunk leak

Can you access the trunk by removing backing from inside the car ( passenger compartment)

try looking at the trunk gasket! if its bad it will let water leak in the trunk as well

The moisture in the trunk is coming through into the car with air movement, since the trunk is not air tight to the car interior.

Let's do the trunk first.
Remove the spare tire and carpet and all else in the trunk and dry it out. I would used small space heater or a blow dryer or anything else that will heat and dry the air. If you want to fix the water leak, you will need to find out where it coming from.
The most common place is the gasket around the trunk lid.
After that, a rust hole somewhere in the trunk.
You may find a new gasket hard to find, and if you do, not easy to replace as well.
Finding a rust hole might require you to take a strong light and have a helper to look either inside when you have the light outside, or outside when you have the light inside the trunk, (night works best).
On older cars, I have often made a permanent hole in the lowest part of the trunk to drain out water leaks.
Make a hole about 1/2" diameter of more, so it won't plug up. There are sometimes rubber or plastic plugs you can remove to drain the trunk as well.
Make it so stuff you replace into the trunk will not cover that hole. Carpet holds water, so you may choose to put a new piece of something in there, or nothing.
Read the instructions below on usuing "Damp-Rid" plastic buckets in your trunk.

Now lets go inside the car.
I recommend you dry out the inside with a space heater or blow dryer as well, for a day or a few days.
Then,
Lumber yards and hardware stores and such places sell a small bucket of stuff called "Damp Rid"" This is plastic tub full of white crystals that absorb water out of the air. Put one or two of these in your car on the floor of the back seat and maybe the passenger's front seat. Follow instruction on the container, and they will remove moisture from inside your car.
NOTE: You must be careful because they work with the top open, and therefore will spill accumulated water when you start and move the car, they'll tip over. If you put them in the trunk, don't forget them there.

Check in trunk for leak.

If your concerned that there may be another place of water entry. Find a shop that has a tool known as the "SMOKER" this is a device that produces white smoke with mineral oil or scentless baby oil. I suggest to fill the trunk up with smoke and see where the smoke escapes to.

hi first check your window rubbers by turning a hose on them it could be a cracked or perrished rubber water will always travel to its lowest point hope this helps

Buick has punched holes in the rear of the car, behind the bumper.
These hole were covered with a tape to prevent water from getting in. As car ages the tape comes loose and the spare tire well fills with water.
NOW THE BAD NEWS.
The rear bumper has to be removed to get access to the holes, for you to cover them. I recommend that you use Extreme Weather Foil Duct tape to cover them. (available at Home Depot)

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.

Easy to fix. Remove trunk liner or peel back on passenger side. Squeeze tabs that hold in the vent housing and push out. You have to loosen the bumper panel over in the wheel well area. This is just two bolts that take a 10 mm wrench and a plastic body clip. Remove vent from outside of car. Take off old gasket and throw away. Clean vent and metal around where vent goes completely with lacquer thinner. Use a product such as Lexel to caulk around where gasket used to be. Be liberal with caulking, it all covers. Reinstall vent and other parts in reverse order. Poblem solved. One hour to do and no special tools required.

Try looking for a strap in your trunk it should be located at the upper inside of your trunk, there could be one or two straps, when you pull on these straps they act as a release lever for your rear seats, pull the straps toward you and push the seats forward from the trunk side.