If you are getting water in the driver's side of your trunk check the gasket on the radio antenna. This will cause the area behind the rear wheel to rust.

Clean out the trunk well, fix any rust, and then coat the entire area with linseed oil. It will flow into all the small cracks and doesn't harden. I just went through this with my 115. I had a 30 year old trunk seal and bad as it was it didn't leak mostly because of the shape of the lip where the seal mounts. Water can't get up over this lip. It just runs around the outside of the seal and drains out along the bottom near the trunk latch.

Cars with a heated rear window tend to rust out where the electric wire come from the trunk into the window. In my slc the wire comes into the trunk on the passenger side. On the 115, even though the chassis are the same, there was a wire on both sides. If you have rust in these areas you have to remove the rear window to fix it properly. Use this as an exuse to install a new gasket.

This is also a MAW (might as well) situation. Since you already have the rear window is out check your headliner. If it is old and faded and you have always wanted to replace it you are half way there since you have to remove both the front and back windows inorder to change the headliner. Since you are going to take out the front window you MAW change the front gasket. It just never ends.

If you go this route search the site about repalcing the windows. I did both in my 115 and my slc. There are some tricks even in you get a 'professional' to do it.