left rear turn signal not flash

Hi I'm finding help to solve left rear signal problem. I have a 2011 ford focus,
symptoms are:
(1) when left turn, there is double click sound from dashboard, left rear signal
(red) does not flash, always off, left front (yellow) flashes at double speed.
Nothing wrong with right rear or right front signals.
(2) when brake, left rear signal off all the time, right rear is normal.
(3) when unlock car, both left and right signal light up
(4) when lock car, right rear signal flashes once and goes off, but left signal
directly goes off without flashing. Front light are normal, they flash once and
go off.
(5) when turn on threat signal, all signals flash at normal speed except the
rear left, always off, no flash.
(6) when pull out the key, before locking the car, both rear signals light up, no
flash.

With these symptoms, what could be the problem? I originally thought the left
rear bulb need replacement, but it can still be lighten up sometimes so it is
good. I doubt it's the flasher but not sure. Do left and front signals share a
flasher? or they are individually controlled by different flashers? Where can i
find the flasher for left rear signal? I do not think it's a fuse problem since
signal can be on sometimes. Is it possible the relay problem? I'm not
knowledgeable about cars. (flasher=relay? I do not know)
Thanks a lot guys, without turning signals, it is dangerous on the road.