Audio Systems: stereo problems, loose connections, passenger seat

Question
I have a GMC 1985 truck.  The stereo (after market) works but cuts itself off when I reach a certain speed, then, turns back on when I hit the brakes.........please help

Answer
Check the speedo is not connected to the radio and the correct feed wires have power, best to have a friend in the passenger seat with a test lamp (12v) or a meter and see what it happening on the two 12V inputs (live and Accessory)
and there are no loose connections.