OK so a light touch of the brakes when the truck is moving or not doesn't support the theory of deceleration and loose wires... Without actually hearing the radio and how it changes from bad to good it's hard to point you at a potential cause. However, does the problem "sound" like something resulting from reception (e.g.: antenna), or more like a short ? In other words, could it have something to do with the antenna, the cable or the connection from the antenna to the radio ? Depending on the effect on the radio, you would be looking at 2 different types of problems. Is it a factory radio or aftermarket ? If it's aftermarket, maybe the installer was not careful in how he ran the wires and left them loose enough that there has been some chaffing... Probably the easiest thing to do would be to carefully look at the brake pedal and associated linkages to see if there are obvious signs of wires for the radio or the antenna cable itself that have chaffed and are shorting to some part of the brake pedal linkage. When you hit the brakes, the (metal) part making contact moves and the short is eliminated....
Kev