something that has happened a few times in my wiring repair experience that had me scratching my head until it was noticed. the plastic or fiber separator for the two wires has a tab that is supposed to index in the socket to keep the wires separated and in the proper position for each function, turn signal and running lights. i have found that sometimes, that tab can become worn, and believe it or not, vibrations can cause that separator to slightly rotate, causing a problem that mimics a bad ground. cheap, imported sockets often times bend the plastic when inserting the bulb, causing the separator to rotate. it is a combination [particularly with these type of sockets] of cheap components and a sloppy locator slot. just something else to investigate.