"Zwo" ist not really a military code, it is very often used in everyday german language. It is used because the real word, "Zwei" (2) , sounds very similar to "Drei" (3). So in situations where it is important to understand the number, you will use "Zwo".

Example:
-"My phone number is 84328" would be:
"Meine Telefonnummer ist Acht, Vier, Drei, Zwo, Acht."

But it is not the "Zwo" that is interesting, but the word "Links".
He sings "Sie wollŽn mein Herz am rechten Fleck", which can also be translated to "They call me a fascist", since "Rechts" also means conservative or fascist. So, when he says that his heart beats "Links", he means that he is not a Nazi, like many people say, but a socialist. It is a very controverse song, since the music does not sound like a socialist song, but rather like a Neonazi song.