Just curious how they're secured to the trunk so I can plan accordingly when I add things to the underside of the trunk lid.

 

on my stock spoiler if you take off the cover under the trunk lid, there should be some long screws that you can take out using a philips screw driver.

 

I've had my spoiler removed on my '89 for over a year now. Upon inspecting it, after you remove the cover inside the deck lid, there should be four nuts right below where the third brake light is which holds it in place.
The ends of the spoiler are held on by plastic plugs which just pop off. It will probably be a bit more difficult than that considering I saw some double sided tape on the edges of the wing. Other than that, there were no screws on mine.