Hello Patrick, Hello Gerard,
The youtube video is a JOKE video that a good friend of mine "created", whem he was in USA, helping a local Audi 100er enthusiast to repair his other
clean Audi 100LS... So, don't take it for a REAL barnfind car... hahahahahaha
The red Coupe S "cabrio" you have seen on the photos is NOT a Karmann creation... You mixed this with the real Audi 100 LS Cabriolet prototyp (which is still existed!) which this car was made by KARMANN...
This red "cabrio" is made by a German 100er enthusiast almost 20 years ago...
There is also another custom made red 2door sedan converted to a cabrio in Germany. Its owner had the complete TOP COVER system adopted from a BMW e30 cabrio. The result is quite successfull with good result, opticaly. I don't know if the body had strengthened properly... But the idea of adopting BMW spare parts to an Audi 100er, is not my favorite...
My old result of a quite deep research I had done to this issue of converting a Coupe S to a cabrio car is more RADICAL...
I found out an almost perfect fitment could be achieved by using a Porsche 911 cabrio (1985-1995) models. You will see that the doors' windows top ending is quite the same...
In addition to this, when the top would be closed. the outline of the COUPE S curved roof line is still remaining...
But there is a HINGE!!! The body floor should be SHORTENED almost 15cm as far as I remember... In that way, the 911 top, would be a "perfect" match to keep the roof line with the trunk cover in the same curve...
So, you can make HISTORY creating a short based CABRIO with Porsche 911 cabrio top...
The short based conversion has also the benefits of creating a more stiffer construction and more easily manufacturing all the extra steel strengthening parts to weld them on the body...
My 5cent,
Theo