alternating flashing led taillights

We are attempting to add two LED oval taillight to our fire dept's grass rig to use as alternating lights in the headboard of the flat bed.  We have them wired to use the "bright" brake light side, we attempted to use a school bus flasher to run them but it won't flash them.... is there a different way to hook this up or another flashing unit we should use?  the flasher was grounded , the lights were grounded.

Thanks,  David