Ok, water gets into the ignition coil on the car, AND it's because a screw is the wrong size that fits the coil cap onto the distributor cap. I have 1 small, fine thread screw and another longer thread and longer screw still only maybe 3 mm.
AND I open the trunk to pull out the tool box and there are 2 1/2 inches of standing water!! WTF??

 

Check the weatherstrip between your hatch fill panel (the spot the spoiler goes on) and the window. If it's old, it's shrunk, and you'll probably see a gap between the two. Use a blade to cut the old strip out at a backwards angle. Then, use up a tube of "black silicone" to fill that angle up. You can over-fill the area so it protrudes out onto the glass. When the silicone dries, cut it at a "normal" angle with the blade, so the thickest part of the silicone is against the bottom of the window, and the thinnest bit is against the top of the fill panel. That should help.

 

You think some new weatherstripping would do the job?? I'm tempted to dump this old junk from the Reagan era and plop down $40 if necessary. Don't want my interior stinkin.

 

Well, I think you'd have to remove that rear panel from the window glass to change that strip. Did I explain the strip I'm talking about correctly?

While your hatch is SHUT, look at the window. Look at the rearmost part of the window. You know that short horizontal panel between your hatch window and your taillights? The bit of weatherstrip I'm talking about goes between the hatch window and that short panel. Spoilers are usually mounted on this panel.

It could be the regular weatherstrip around the inside of the hatch, but give that spot I'm talking about a quick look. It's happened to my '86 twice, I'm sure it's shrunk back on your '84.

You could always do what I did to find the leak, too. I looped a hose around my car so it hung down over the roof, and onto the window. Then I dove into my car thru the backseat, and tried to see where the water was coming from.