The left turn signal blinker on Toyota works sometimes and not others. It is weird, it seems when the weather is cold, it works, warm weather it works sometimes.
The car is: 2002 Toyota 4-Runner, limited
If by not working you mean you switch the switch, but hear no clicking sound at all, that is probably the switch itself. If you hear a fast clicking sound but none of the turn lights are lighting up on the car, it could be a lightbulb that is failing, or not making a good connection.
Be sure to check the fuses for the turn light signals too.
There is really only one cure for this insidious problem. You need a new 4Runner, now. The next two generations are so much better, any excuse will do !