First off, any vehicle being used for surveillance must have tinted windows; for many reasons, but mainly so other people and/or the subject cannot see when the agent is recording or documenting them. Also, if possible the goal is to make it seem as if there is no one in the vehicle while conducting surveillance. Many times the investigator's vehicle becomes like an office, and one does not want people that are acquainted with the subject and/or neighbors seeing them while going through files, writing things down, video taping etc. Naturally, the object is to be as subtle as possible and the least people can see the better.
We believe the best possible surveillance vehicle is a mini van with tinted windows that is not too bulky in size. These types of vehicles are excellent when it comes to almost any case. For example, let’s say our client wants to catch their girlfriend cheating. If she looks back in her rear view and sees our agent a time or two driving in a minivan she will most likely be less suspicious than if it were any other vehicle. The reason for this is most people associate minivans with families, soccer moms, etc. This being said, they draw the least amount of "investigation attention" to them. Honestly, do you think someone cheating, committing fraud, or carrying out some other form of illegal activity would prefer a Dodge Grand Caravan, or Dodge Charger parked outside their house?
(Note: I used this example of a Dodge Charger to point out the fact that no investigator should ever use a vehicle that looks like or resembles a law enforcement vehicle. This is because a lot of times the subjects being followed are paranoid and looking to see what sort of vehicles are following them, parked near them, etc. and if they see what looks like a law enforcement vehicle it may prevent them from doing what you are there to document!)
The vehicle must also be small enough to be able to maneuver through traffic, and versatile enough to park in any space that a car can. An investigator's goal is to have all possible outs covered i.e. if the subject has E-Zpass and can go through a toll faster than the rest of the traffic, he must also be able to. This is the overall goal in fact: to always be one step ahead of the subject, prepared and equipped, and have the ability to continue documentation under any arising circumstances.