Heinz,
very interesting...and there is nothing wrong with you English! If only I could write in German as well..
Anyway the interference you describe is certainly due to bad contact in the wiper motor. But it shouldn't impact on the radio: many things in a car make interference in the radio spectrum, the spark plugs being the worst but the alternator and heater motor also cab contribute.
I suspect that the radio power feed is shared with the wiper motor, either the positive or the earth.
You could, as an experiment, try running a new power feed directly from the battery to the radio. If that removes the noise then the answer is simple..
If it doesn't then try checking all the earth points under the dash.
Another possibility is the antenna earth:it should be earthed to the panel to which it is mounted, but given the structure of the car isn't all metal it is worth running a ground wire from the base of the antenna mount back to a known, good earthing point.
Finally, back in the 60s when I was helping Dad fit a radio to his car he fitted a smoothing choke/filter to the power supply. I've no idea it they are still available, but they work as a 1:1 transformer, supplying only smooth 12v to the radio.
A quick web search and I found this:
Smooth Power Not difficult to make up and sure to supply clean, smooth and noise free 12V!!!
