What Are Those Things?

I´d guess that about 20-25% of the trucks and buses in Brasil have these things installed. I saw them on my first visit to Brasil way back in 2005 from the window of a moving bus.  I asked people off and on what they were, or what their purpose was and, effete snobs that all of my Brazilian friends are, nobody ever knew.

Then I´d forget about them until, once again, I´d be on a bus on the highway and I´d see them  again, and wonder all over again.

It took almost 5 years before I saw them closely enough to know what they were, and then I realized that I should have been able to figure it out almost immediately, if I´d only thought about it for a bit.

Can I assume that everyone else knows their purpose?