Am car radio not working

I have a Ford pick up truck that the am radio does not work with the engine running. It works on the accessory when the key is turned to it, but the minute you turn the key to the on position or with the engine running I get only static. The plug wires are good and the plugs as well. I can't find a noise suppressor in the system. what should I do? Antennna and cable are new. thanks Jim