Usually in this case it is a bad bulb or abad connection,or a bad ground.If you have a voltmeter or a testlight,check for power at the light socket,when signal is turned on ,and go from there.If there is power,it colld be a bad bulb, or a bad ground. Clean the connections and make sure the ground wire is connected.If there isnt power,trace the wire back ,look for broken wire or dirty or loose connectors.