Driver side rear turn signal is hyper flashing very fast?

My truck, driver side rear signal is hyper flashing very fast. I replaced the complete signal light with a new one since there was water inside. After replacing the new light system, it is still hyper flashing. I removed the right back taillight and connected it to the left side but it hyper flashed.
It works fine on the right side so I re-installed.

I have attempted to locate the turn signal relay to see if it is bad. But looking at the diagram there are no relay signal listed, but other relays are listed for what item they control. I turned the left rear signal on
again, and as it hyper flashed I took a long screwdriver and attempted to
hear the relay open and closed a long with placing my finger on all the relays one at a time and I could not feel or hear a relay working. I am checking the fuse box in the engine compartment. Everything I checked showed the relay for the rear signal lights is located in the engine compartment. I have a Duramax diesel engine. It has to be running for
the signal lights to be checked unless there is another way to perform this task. I checked my owner's manual and the relay is not listed. I may
be wrong about checking the Engine compartment fuse box is the correct one to check. There are two others inside of the cab.

Need help.
David