Tailight failure when braking

So I’ve recently replaced my taillights with aftermarket. I installed some Alpharex and started to have an issue where when braking, the left tailight is not signaling. I’ve since replaced those tailights with DEPO tailights and I am having the same exact issue! Yikes. Interestingly, the DEPO tailights come pre-installed with wiring going to the bulbs. This tells me that my issue is located somewhere between the main plug at the tailights and forward.
Hopefully this is clear. How can I start to diagnose and fix this?

 

Get warrantied...

 

Remove the lights, and swap them side to side for troubleshooting.
Even supported on a towel, you can verify whether it is the light assembly or the truck.

If the fault moves with the light, the light has issues. If the fault remains on the same side, the trucks wiring or something is the issue.

 

Great advice.

 

Edited the 1st post. Issue seems to be wiring of the truck. I suppose the next step would be to use a multimeter starting at the origin and testing along? Naive to this and greatly appreciate any help