Hitchhiking on the back of a Pick up Truck is illegal in Southamerica as well as in Germany. The only difference is, that the people do not really care about that fact.
Especially if you´re in the middle of nowhere, and that´s where you are most of the time, when you are traveling in the south of Argentina and Chile.
So do not hesitate to ask them besides the road if they could take you for a while (and for a while means at least 100-500km in their country 😉 )
Especially in Chile, Argentina, Peru and Ecuador it was easy to get a ride from one of these trucks. Obviously it is not the safest way of traveling but for me it´s one of the best:
You are fast as a car, sometimes you can make it comfortable as a bed and you enjoy a 360° view with fresh air (you should not drive around like this in the big cities) all around you.
So every time you see a Pick up Truck, put your thump out!