All indicator lights on one side of the vehicle, including the trailer or semi-trailer, are switched and controlled via three internal power or turn signal control circuits. One control circuit for the tow vehicle front turn signals, the second for the tow vehicle rear turn signals. The third control circuit is for the trailer or semi-trailer indicator lights. The error indications of the first and second control circuits are given through the light indicator of the towing vehicle. Third control circuit error indications are given via the trailer indicator light. The negative influence of long cables on the control characteristics is very effectively reduced with the three-circuit system.