There are small electronic modules you can install on the 3ed brake light circuit that will cause the light to flash repeatedly and then go steady. They can be programmed for different number of flashes and different flash rates. I see these commonly used on motorcycles, occasionally on cars and trucks. There are many different ones offered on Amazon.
Also, get an LED strip that goes across the truck under the tailgate. These add a lot of light and are quite effective at getting attention.
Lastly, if you have a cap, topper or hard tonneau cover that has a third brake light in it, hook it up if it is not. You will find a short pigtail sticking out the harness on the LH side near the spare tire, which is for powering this High Center Mounted Brake Light, AKA 3rd brake light. If you have a cap or other installed item on your truck, that obscures the 3rd brake light on the back of the cab, some states (Texas) will cite you, if you do not have a visible one on the cap or truck body.
http://www.3rdbrakeflasher.com/instructionshttps://www.atcomp.com/services-view/recreational-vehicle/This is the correct connector to plug into a LEER or other brand topper wiring to hook up the 3rd brake light.
https://www.ebay.com/p/American-Technology-C90-907-Center-Mount-Stoplight-Wiring-Kit/567842466https://www.atcomp.com/wp-content/uploads/2018/08/C90-907.pdfCharles