The easiest place I found to accomplish this was in Le Havre. Across the small street from the garage, standing in the backyard is your kissing target. If you walk west down the street, you will come upon a group of three permanently stationed soldiers. Get within view, pull a weapon and, without hitting anyone, pull the trigger to set off the alarm. Keep your eye open from which direction the responding Sturmwagen comes from. If from the west, feel free to run full speed into the backyards on the north side of the street back to the green dot. If the Sturmwagen comes from the east, pace yourself to make sure the vehicle stops west of your destination to make sure you have those few necessary seconds before the troops can see the girl and ruin your escape.
Simply wash, rinse and repeat until you hit 50.