I'm going to get this thing done, but the mounting points are a bit trickier on a convertible. Not as straight forward as mounting to the seat belt harness. I know there is a tech article here on this but there aren't any pics. Just thought I'd throw a couple of pics of what a cross bar looks like on a C4;

 

here

 

There was a post about doing that a while ago. Don’t remember if it was ever done, but I remember the discussion being about that it wouldn’t be that beneficial.

 

Yes...revlimit constructed one as per the tech article and found it very benificial. GM designed rear cross bracing on the C4 and 4th gen convertibles that braced the wheel wells. As I have a vert, I will prob design one to attach the b-pillars where the front seat belts mount and also one behind the rear seat to the posts where it locks/unlocks.