Turn signal sometimes works

On my 1999 GMC Suburban, the left turn signal sometimes works and sometimes does not (right works no problem). If I hold the lever, it works most of the time (I have to keep holding it) but if I just push it to the left, it does not work. As it has gotten colder, the truck usually needs to warm up for it to even work at all (it seems).
Any adivce? Easy fix or do I need to take it in someplace?

Thanks!!