You might check the lights. If one or both are burned out there will be no current to drive the flasher. Also look for a bad ground, they can cause all sorts of funky symptoms. The reason others have asked which switch you are using has to do with how they are wired. For example, mine is a 4 wire switch. It needs separate turn signal lights. Not integrated with the brake lights. Other switches will integrate with the brake lights and they are wired differently.
