Left turn signal blinks fast in rear and regular in front

Very simple the bulb is out. When the blinker does that its telling you the bulb needs to be replaced

There is corrosion in bulb socket. With lights off using a flat head screwdriver scrape the contacts on the inside of the socket. If this doesnt solve problem socket needs to be replaced.

the bulb is blown out. you need to change it. also if the bulb is not blown out or if the blinker still blink fast after you change the bulb then you have a ground problem. just remove that blinker light and check the ground wire on it. if you need to replace it and find a good groun on the car. you should be fine. this is all it is .