If I remember correctly very basic ones started to get put in along with airbags to see if the airbags were working correctly.(Deploying at correct speeds/impact dirrections etc)
It's tiny, it's just a newton meter and a direction sensor..(not sure how they're called) basicaly it's a device that measures the force and the direction. (The most basic old kind is a small lead ball on a spring sitting on a plate. The location of the ball on the plate is recorded, if you push the thing to the right the ball will be on the left side of the plate and extend the spring relativley to the force of the push) I think Nissan used to put them in the OLD 350zs for the "coolness" factor the electronic dash used to display the gforces and direction...todays electronic sensors are tiny, the Segway is loaded with them.
I'm not 100% sure if US cars have them, nor am I sure how you'd find it. It'd just be a small pastic box and it could be anywhere. It's possible that if they really put in black boxes you'd have multiple sensors, for example to record the distribution of force through the car from a 45 degree angle impact into the rear right corner.
The "suggested" black boxes of the future, with the GPS would need some sort of line of sight to the sky. Which means either a small knob on the roof, the top of the dash, or under the rearwindshield. So you could track it through that, but as far as I know NO car built today for the US has a GPS system that uses the position of the car for any purpose other than route keeping etc. No GPS system that I know of "sends" signals anywhere.