Right. To further define "design intent" the following personal opinions may apply:...
[b]Sport Utility Vehicle or Van, unibody construction: passenger and cargo transport, moderate trailer towing?
[b]C1 through C3, first three generations of Corvette, body on ladder frame construction (plus big V8, rear drive): impressing members of the opposite gender
I agree that a Cadillac convertible is not likely designed with the intent to tow, but neither is a Corvette... the construction method (unibody or body-on-frame) isn't a great single indicator of capability, but a look under the back at specific structural details may provide some indication.
One hint would be the availability of hitches. 2" hitch receivers with capacity well beyond the vehicles rating are readily available for common front-drive unibody minivans, and simply bolt into the six provided threaded mounting points; on the other hand, I can't find a hitch anymore for our Honda CRX, and even when they were made their attachment system included some sort of clamp onto what was supposed to be a tie-down point.