I have a registered trademark in my country of origin, could I use it in a Mercosur Country?

Trademarks are subject to the principle of territoriality; therefore a registration only protects the trademark in the country (or region in the case of the Community Trademarks) where it has been registered.

Regardless of which, it is possible to use a trademark in any of the Mercosur countries without registering it, unless it is confusingly similar to a prior trademark.

In this case, you should take into account that non-registered trademarks have little (or no) protection. Therefore it will be almost impossible to act against counterfeiters.