When I turn on my left turn signal it blinks once, and that's it. My right one works fine. What can cause this? Thanks

 

Have you checked both front and rear bulbs for a failing light bulb?

 

i had a similar problem on my early 2000, there are two flasher relays in the compartment under the steering wheel, try swapping them.
That was my problem, one of the flasher relays were bad.
Now it depends what date your truck was built how hard it will be to get the flasher, they had a running change, early models used a black flasher and later a yellow they will not interchange.

 

Thanks it was the flasher relay. I swapped them and it did the trick. Its the yellow one I just have to find one.

 

yellow not that bad, the black are much harder to find