Guys With Factory Go Wing Cars Please Look Here <<<<<<<<<

First place auto products offers a molded template for the cut out, for the Duster, Demon, Dart Sport decklids, to simplify this process. These templates fit the underside of the decklid exactly for a precise and factory correct location and cut. They're $30 a pair and can be used over and over again. Here's a link.

http://www.firstplaceauto.com/catalog.htm

That's some sound advice, as I'm just getting back from trying to take some measurements, and due to all the contours it is extremely difficult to get anything remotely close. Short of removing the bracket, and taking measurements from the bolt holes.

Your aftermarket bracket may aslo be different...

These are pics from the driver's side bracket, note the the small square opening, it is not on the same center line as the larger opening.

View attachment 77519

Here is a measurement from the trunk lip (4 1/2")

View attachment 77521

And the width of the opening (2 3/4" wide)

View attachment 77522

The rest I really couldn't take a good measurement to give you (short of removing the bracket).

Short of "Convertriple's" solution...I suggest to drill the holes at the locations posted earlier, and use the opposite bracket (flipped over) as a template to cut the large opening.
Then use the bracket for small holes...

I tried,
Giles