A cop touches your tail light to leave fingerprint evidence in case she needs to prove that a stop occurred. It's also a safety precaution, since it's generally safer to approach from the side of the vehicle than from the rear.
It can make you feel extremely nervous when a police officer performs a routine action like touching your tail light. You might wonder if your vehicle is damaged or if the officer is extra-suspicious of you because he thinks you've done something seriously wrong. The goods news is that tapping the tail light has very little to do with the reason why you were stopped. It's more about protecting the police officer than incriminating you.
TL;DR (Too Long; Didn't Read)
A cop touches your tail light to leave fingerprint evidence in case she needs to prove that a stop occurred.
Touching the Car Proves the Stop Occurred
When a police officer touches your tail light or bumper, she leaves fingerprint and DNA evidence behind. This provides critical evidence so investigators can place the vehicle and its owner at the scene in case you try to flee. Tapping the tail light establishes a physical connection between the vehicle and the police officer. This is especially relevant if the police officer is attacked or injured during a routine stop.
It's a Safety Precaution
So why the tail light? Why not the trunk, which is larger and far more easily accessible? Tapping the tail light keeps the police officer to the side of the vehicle and not directly behind it. This keeps him out of harm's way should the driver start to back up.
Approaching from the side is also the best position for an officer to see the driver's lap area and the back seat to assess whether any weapons are visible. This is especially relevant for one-officer patrol units when there's no second officer to maintain a safety watch on the occupants of the vehicle.
Cops Can Spot Evidence in Plain View
It's not unusual for people to try to hide illegal drugs or weapons after they've been pulled over. Tapping the back of a car helps to distract criminals and stop them hiding something.
Generally, a police officer can only search a car if you consent or if the officer sees suspicious items sitting in plain view. If the police officer distracts you before you have time to stash the goods, the police can seize the items and conduct a more thorough search. The police report an increase in arrest rates for intoxicated drivers and those found to be in possession of prohibited substances and unlicensed firearms when tail light tapping techniques are used.
Old Habits Die Hard
It's also possible that a police officer might simply tap your tail light out of habit, particularly if he's been on the force for a while. There are traffic cameras all over most city streets, and most law enforcement vehicles now come fully equipped with cameras and other technology that can prove that the police officer was present at the scene. Touching the tail light has become an obsolete, unnecessary step.
There's also an argument that tapping the vehicle exposes the police officer's location in relation to the car, making him more vulnerable to attack. Some police departments now discourage police officers from tail light tapping.