Rear Wing Spoilers
  Adding a rear wing spoiler to the trunk of your W-body can visually 
  enhance its appearance and give it a more aggressive profile. On a traditional 
  rear-drive race car, a rear spoiler channels air around it to provide downforce 
  on the rear end, which increases traction at high speeds (>100mph). On a front-drive 
  vehicle, a rear wing is just decorative. However, they can also be functional. 
  A rear wing with a bright LED brake light will improve safety, as properly functioning 
  LED brake lights are brighter, more noticeable, and have a 0.2 second faster 
  response time than normal brake lights. The 0.2 second difference may sound 
  very small, but it could mean the difference between a close-call or a disaster.
 See ground-effects kits 
  & rear wings for...
  First Generation W-Body 
Second Generation W-Body
If you can't find the right rear spoiler, you might look into buying a universal one.