1995 Oldsmobile Ciera Left Directional Not Working; Right D

Emergency flashes work properly. All other lights work properly. Left directional signal indicator light on dashboard solid. Left rear and and left front directional lights solid (not blinking) Exaiming bulb in left rear unit finds both filaments lit and neither blinking. Right directional lights front and rear working perfectly. Right directional indicator on dash blinking.
Thanks for the prompt reply. I must add the following bit of information: This problem started appearing in 2005. Initially, giving the left rear tail light assembly a sharp rap was sufficient to get the bulb blinking. Eventually, changing either the front or rear right side light bulb got the job done. Now, neither rapping the tail light unit or changing the bulb solves the problem. Knowing this additional information, would you still assume that the problem is in the tail light flasher?