Lincoln turn signal problem

[Stem]

Hi all!
My friend got a 67 Lincoln Continental and has troubble with the turnsignal on the rear left side, he get's both front to work at all times but he ony get the right one to work when normal driving light is on, on park it doesen't work. On the left side he gets the indicator in the dash and on the front fender/bumper to work at all times but not the rear. He has concluded that there are no faulty whiering or fuses damaged, and that the bulb is okay......
Tips???