Usually when this happens the bulb which does not come on is faulty. However, if water gets into the light it can adversely affect the bulbs' socket. Check the socket for water damage. If damage is found replace the socket and repair the lights' cover, as it is cracked or coming loose from its housing.
Also, use a test light (see link below) to check for battery voltage (12v) on the DK GRN (dark green) wire metal connection of the bulb socket, with both, the ignition switch and turn signal switch ON. Next, check the BLK (black) wire (this is the ground wire) of the socket (using the test light).
If either wire fails the test, check for voltage (or ground further down the wire) there may be an open or ground in the wire preventing voltage (or ground connections) from reaching the bulb. Repair the wires as necessary.
Monday, August 27th, 2018 AT 7:58 PM