Radio links were tightly regulated. Part of the Nazi propaganda effort was full control of the airwaves, and making sure everyone had access to them, so they produced two types of very cheap receivers German citizens were encouraged to buy.

Germans were only allowed to listen to Nazi controlled stations. Being caught listening to overseas broadcasts such as those made by the BBC in London was forbidden and could result in imprisonment.

As for letters, I believe that letters had to basically go on a meandering journey to Spain before finally making their way to their intended target.