Speed sensitive volume is a feature they included, even tho four channel audio and a real equalizer would have been nice to have as well.
Anyway, the Brake Controller provided speed data to the amplifier for the sound system, that's what the wire the arrow points to does. You can upgrade to a better AM/FM head, if you leave the factory amp in there, the feature works with the new aftermarket heads. You can then do the four channel mod and get true four channel sound from the existing system (the amp in our cars is four channel, it's just that the factory head is two channel).