Opening from a Karmann Ghia engine lid has to be done from the inner side of the car. This can be done with the use of a cable which can be controled by a button that can be found on the panel under the rear seat. This cable goes through the rear panel and via a tube to the engine lid inner lock. 

In many cases, this cable can be heavily damaged or even broken so it isn't any longer possible to open the engine lid. Better is to prevent problems and to replace a corroded or, due to wear, damaged cable in time.