Heres something else, with the rear left bulb removed the front still does the same thing... the front bulb lights brighter like brakes are applied and then stops flashing.
yes it's the connections/sockets in the left rear. clean the connectors really well. you can get tarnish off a few ways, a small piece of emory cloth works well. you can swap the left/right bulbs to see if it's just the bulb.