If the socket is corroded inside (Whitish and crusty) There is the possiblity that it is conducting between the terminals (assuming it is an 1157 bulb) hence no load from the tailight to the flasher and it flashes quickly. Also, ground faults can cause electricity to do crazy things. I would think it is safe to assume that that bulb grounds to the chassis metal via the taillight housing or, a wire. Napa also has the capacity to print you an entire vehicle ground diagram. Really handy if you have ever worked on a Ford with the standard 192,787,121.799 Body grounds. Also if you have an ohm meter, with ignition and lights off, Ground one lead, and try the terminals in the socket individually. It should be infinite resistance.