flasher fuse keeps blowing
 

My friend has a '81 Gmc Sierra Classic 1500 with a built 454, the problem he is having is that any time he puts the truck in gear it will blow the flasher fuse. He can change the fuse, let it run all day long if he chooses to, but the moment he puts it in gear, it blows. Does anyone out there have any ideas, it's frustrating as hell, I can tell you that. Thanks for the help.