If you dream you forgot where you parked your car, it is a dream of anxiety. You probably had an anxious, stressful day and the thoughts carried over to your dream. You may have experienced the problems of parking somewhere and you forgot the spot where you parked your car. You became worried and this experience made a mark on you. When you get anxious, old memories of stressful events surface. Your brain then uses one such memory to replay in a dream.