Hello my name is Dave.
It sounds like you could have a faulty turn signal switch or the circuit from the brake light switch to the turn signal switch. The way the brake lights work on your vehicle is that the third brake light (center) has a direct circuit from the brake light switch to the light. The regular brake lights double as the turn signals so they are routed through the turn signal switch. What the switch does is re-rout the power to the turn/brake element in those lights depending on use, in other words it disconnects the brake light circuit when your turn signal is on, that is why you only have 1 brake light on one side when you have a turn signal on. I would check the wiring to the turn signal (multi function switch).
I am attaching diagrams for the exterior lights of your vehicle, and I know you said you don't have a multimeter to use for testing the wiring but I am also including a link to our tutorial for testing wiring, I am concerned that the issue is the wiring going to the turn signal switch since that is a more likely cause for both sides to fail. Here is the link to our tutorials, both for testing wires and using the multimeter:
https://www.2carpros.com/articles/how-to-use-a-voltmeter
https://www.2carpros.com/articles/how-to-check-wiring
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Wednesday, July 28th, 2021 AT 1:34 PM