Turn signal problem

Typically when symptoms change between head lights / running lights on or off, it's due to a bad ground connection in that corner of the vehicle. To verify that, first check if all the running lights work with the head lights off. If one does not, suspect its ground circuit. An additional clue is to turn on just the malfunctioning signal with the running lights off. If there is a bad ground, current will go through the bulb, then find an alternate return path to ground by going over to the same bulb on the other side, then through its ground circuit. You'll see both bulbs are on but not at full brightness. If you remove the working bulb from its socket, the malfunctioning bulb will go off too.