Fast blinking signal light turn

We have a 09 impala, had a left turn blinker that blinked fast. Husband changed the bulb, checked the fuse. . So he tried everything but nothing was working to fix problem. We just bought this car and didnt notice the interior light fuse was missing. That was the problem so husband put a fuse in the spot and it stopped the fast blinking and it's working fine and we now have interior light too.