Take left rear bulb out of socket. You will see two spring loaded contacts. Use an Ice pick tester. Clamp tester alligator clip to a good bare metal ground place near by. It's VERY important you are sure that this clip has good ground. Turn on ignition switch, push lever on column down. Go back and touch contacts down in socket, one at a time. See if one of them blinks the ice pick light. If one does blink nice and steady, move ground clip to touch just the socket while touching same contact with pick. Be careful not to touch clip to pick.
Report.
You have provided good info for us to determine the probable cause and now you can pin point and confirm.
The fact that the brake lights work, which use the same filament as turn, and tail lights work, seems to show it's a weak blink input problem, not a ground problem. These test may help determine that. You switched bulbs, so not that.
Other causes for your issue: (blink input/output)
1. Dirty/corroded/broken socket contacts somewhere, including turn signal switch
2. Loose socket input wires or partially broken wire inside insulation.
3. That 3 wire connector, you mentioned has bad/corroded contacts.
If the ice pick test at socket shows the same issue, next test:
Unplug the 3 pin connector that has three wires going to rear. Do the same test procedure on the three contacts in the live half of connector. Look for good strong steady left rear blink signal on one of the pins.
Report. If this test has the same issue, we will have more tests/recommendations. No sweat.
Just to be sure, not being picky, but your comment "Verified that wiring to light is the same as right side that works OK." Verified by what method? Wires are not the "same", they are different wires with different colors. from different connection points along the way and different switch feeds. This has nothing to do with you, it has to do with troubleshooting from our house....BUT... We mostly have to ignore statements like that or we will miss something. Let's stick to testing a little at a time. Concentrate on doing good thorough tests and we will catch the gremlin. Did you get the ice pick tester yet? Get a voltage tester, not a circuit/continuity tester.
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