I meant to try and get the light to blink properly. Put the ground to negative, the tail light wire to positive, and then take the brake like wire and flash it on the positive. If they don't work like that, it's the lights. If they do respond to that, it's your truck wiring.
Well, what I meant was that the lights are either on or off. The bulb itself is a dual filament with a ground, and a hot for each bulb filament, the running light and the brake/blinker. The flashing will only occur when the blinkers are blinking. On off, on off. My guess is it has nothing to do with the trailer lights, but to do with the trucks wiring.
Do you have another trailer to try to hook up to, to test to see if you get the same results? Go to the trailer place or hook up to a friend's trailer and give it a try. They also make a little plug that tells you if everything is working properly, try that.
Again, at this point, I'm sure its to do with the truck. Not a flasher or anything but a misplaced wire or something.
I'm no expert, but have rewired (and currently re-wiring) a few trailers and added electric brake controllers on trucks as well.