Check out the Rontan M6Q36. They are extremely bright and also have dual input. I have them in stock in blue and Amber, but red, white, or green usually ship within a week. They are also very thin, so they will fit in your headache rack no problem. the dual input would allow you to tie it to your brake signal, but also be used as a warning light.
they are $90 each and come with a 6 year warranty.
Below I have a blue one flashing away in direct sunlight, it is in it's included surface mount bezel, but it does fit in a standard 4" grommet mount. Note, video never does lights justice in color or intesity, but it still does a dang good job on camera.
Rontan makes another version called the galaxy36-g36. it is pretty bright too, but it does not compare to the M6Q36. the galaxy's run $72 each but I do not keep stock.