If it only happens when the lights are put on then it's very probably caused by sparking from a bad connection - you can check it out - does the interference stop when the car's not moving? Does the interference happen on sidelights only?
When you have the answers then take out the bulbs in turn until it goes away. You will be looking for a bad possibly corroded connection - don't overlook earth wires to the vehicle body and the numberplate light.
Of course - I could be wrong - I'm just guessing.
Good Luck...
November 2015
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