Gecko has NOT yet said if the headlight on that side is working. That is a key diagnostic. If the headlight works, then the earth is OK. If the headlight doesn't work, it is likely to be the earth.
IF the headlight works... I'd be getting out a multi meter. Put the blinker on. Test you have flashing 12 volts when you put the meter probes on the earth contact and the blinker contact of the bulb holder. If nothing, test from the blinker bulb centre contact to a real earth (a bolt attached to the body) or to the battery negative terminal. If still nothing, you have a wiring fault - likely to be in the plug/socket, unless mice have been eating your wiring.
If you don't have a multi meter, buy one. Jaycar Electronics have good cheap ones. Also getting a pair of multimeter leads with clip ends is good. You can then fit the black clip lead and a red probe lead to the meter. Connect the black clip to a known good earth or to battery negative, then test for (flashing) 12 volts at the centre bulb contact, using the probe of the red lead. If nothing, then open up the connector and test for flashing 12 volts at the connector pins - the car side of course. If you don't have flashing 12 volts at the wiring connector (but you will) then you have a wiring fault. But as Dmccurtayne said, it will be connector problems or bulb holder problems. Use the multimeter to find out where you have flashing 12 volts and where you don't, that will give you your answer.