Ok, I found the flashing light hidden high behind the glove box.
But I also found something else: I found the problem!!!!
The fuse of the wiper was blown. I did not notice that because usually I drive the car only in the dry and everything else worked.
There must be some connection between the wiper fuse / power supply and the immobiliser resulting in the strange behaviour described earlier. I am an engineer and could not imagine such a relation, but it's a Morgan.....
Thanks again to all for your support and hints.
