How does costums work with rented vehicules?
 

Hello everybody,
I know this page is mainly used for motorbike-questions. Well I actually have been traveling around in South America by motorbike for a couple of months. But now I plan a little trip by 4WD-car. Therefor I would like to know, if anybody could help me regarding this question:
Is it possible and in case so, is it difficult with costums (aduana) to get with a rented car from one country to another and back again. It will just be for short time, means maybe a week in the other country.

I would appreciate your info and recommendations.

Thanks

Burnout1