Left turn signal flashing rapidly

Hello, when this happens and there are no other light bulbs blown out, its usually a bad ground wire, but on this car, there are two options for the tail/turn signal lights. If you go to the left rear and on the first diagram, you'll see the two different options for this car. If the left rear turn signal has a 3 wire plug it is Without the LZ option, if it has four wires like in the diagram where it says "Logic", then you have the LZ option.
Now depending on which one you have, will determine how to diagnose this issue.
Looking at the full diagram I believe the Logic chip is for a warning light on the dashboard that you have a bulb out as well as it is an LED instead of a regular lightbulb.
But almost every time I've seen this happening, flashing fast without a bulb out, it's been a bad ground wire on that circuit. When you turn your headlights on the "Tail" lights come on with it. So that is how this can happen, there's extra current flow due to the taillight and if there's a bad ground wire, the current will take a different path to Ground.
So, check all the black colored wires, Front and Rear and make sure you don't have a broken or lose ground wire somewhere. Follow the black wires from the connector at the blinker/taillight all the way to where it connects to the chassis. Do a thorough inspection of the ground wires. Check both right and left sides, front and back. If you can, use a test light or multimeter to check them. I will put guides up for both.
If you hook a test light up to Battery Positive and touch the test light to a ground wire the light should light up. You'll know you have a good ground connection.

https://www.2carpros.com/articles/how-to-check-wiring

https://www.2carpros.com/articles/how-to-use-a-voltmeter

https://www.2carpros.com/articles/how-to-use-a-test-light-circuit-tester