I have a strnge phenomena which doesn't really bother me much but wondered whether it was unique? The faster I go the louder my radio gets, not exhorbiantly loud, but iitt is noticeable.

 

I think the volume increases as the cars speed increases just like the windshield wipers increase their speed as the cars speed increases.

 

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).

 

Volume increases with speed to compensate for road & wind noise