C&M, if the turn signal in the left front of the truck is working along with the left rear turn signal, then you problem lies in the wiring or the mirror housing. So, check that first. If it is inside the mirror, then I would next remove the bulb and use an electrical tester or multimeter and see if you are getting power to the socket. If you are getting power to the socket, then odds are the socket is toast (been expanded) and you need to put in a new socket. If you don't have power, then I would next go to the triangle piece on the drivers door, remove that and then find the plug there at the mirror and see if you have power there. If you have power there, but not at the bulb base, then your wiring problem lies in the mirror itself. If you don't have power there, then you are looking at a broken wire between the plug at the top of the door and the split in the harness in the footwell area.
