2012 3500HD Turn Signal Always Blinks Fast

Hello, I'm a fairly new member and this is my first post. I searched every keyword I could think of and could not find this topic...

I have a 2012 Silverado 3500HD Crew Cab LT Duramax that had the box removed and a flatbed installed. To put it mildly, whomever did the work did a poor job and I'm working on doing it right.

I have completely re-wired the lighting on the bed, after tracing down every wire related to the on-vehicle lighting. (I am aware the trailer wiring is a different system, and I'm also an experienced electrician.)

Problem is that both turn signals blink slow a few times then fast as if there is a burned out bulb somewhere. Everything is in proper working order. I removed the LED lights and replaced with standard thinking that maybe the LED's didn't draw enough power and the computer thinks there is a burned out bulb. No affect.

I know there are no shorts in the system. I even hooked the tail-lights up to the ground in the chassis wiring harness (one per side) versus just grounding to the frame. Does anyone know how the vehicle determines a wiring issue? Based on current? What range is "normal"? Where is current sensed? The only thing I can think of is that my flatbed with one turn signal has a different number of turn signal lamps than the stock body.

Or there is another issue??

Any help or suggestions would be appreciated. Thanks.