One taillight not working

[sandyrusty]

I have a 1999 Ford Ranger. One taillight is not working. It is not the bulb as I have replaced it and also switched it with the other taillight bulb and it works on the other side. The brake light works, just the taillight doesn't. There is power reaching to inside the socket where the bulb is plugged in. I don't know where to look next?

 

[RonD]

If there is 12volts at the socket but no light up then the Ground must be missing.

A light bulb works by power passing thru it, it heats up and lights up.
If it just has 12volts on one contact no power is passing thru the bulb, if it had 12volts on each contact same thing, no power passing thru it.
If bulb has 12volts and Ground(0volts) then power passes from 12 to 0 and bulb lights up.
(It actually goes from 0 to 12 but that another story)