2000 E450 rear left turn signal not working

I had exact same problem with my 2004 29' Coachmen DS.

changed the bulbs and all the easy stuff. ended up paying the Ford dealer to run a new wire.

It wasn't cheap, but i'm not that savvy with car stuff and I had to get it fixed for an upcoming trip :(

Don't know anything about your vehicle, but check if the bulb is a double element (i.e. one element for the bright brake light, another for the parking light/blinking turn). Switch bulbs from left side to right and vice versa to troubleshoot. Often it's bad contact due to corrosion. Flasher relay should be good if one side works.

Using a different ground, check for power to the socket.

If it shows power, check the ground of the socket.

If it does NOT show power, check the wires/connections/fuses that feed that socket.