I've solved the problem. My radio needs +12V and ground. I used this setting on another car. But here ONLY +12 V cable is required. I could not believe when I found out.
Radio works on both occasions - with or without ground cable. Although I must add that this is a second-hand car and wiring under the dashboard was torn apart with no standard connectors that are usually included!
In the end: I really didn't expect the Frenchman to be so 'touchy' about wiring a radio.
BTW: On another car I used power cable from front lights
. Simple as that and it worked.