I have a Chevrolet cobalt and the left turn signal will not work the inside light on dash flashes fast. I have changed the light bulb still doesn't work.

Did you replace the bulb with the same number as the old bulb? Different numbers will fit in the socket ok but draw a different amount of current thru the flasher unit. This will cause rapid flashing or slow flashing when all the bulbs are good.

Fast flashing usually indicates that one of the exterior bulbs isn't functioning. You've checked front and rear signal lights, right? If the bulb is good, it could be corrosion in the socket, bad ground, broken wire, etc.