Signal flasher on trailer wiring

Hi all. The wiring plug for the trailer is not getting any signal for indicators at all. Traced it back on the weekend and found (what I later discovered) to be two flashers, one on each left/right wire. As I understand it, the signal flashers should be under the dash and I can certainly hear them, so why would there be two other flashes downstream from there in the trailer wiring? Isn't the flasher meant to have contstant voltage in, rather than intermittent? I was thinking perhaps that is why its not working, it does click, intermittent signal goes in and doesn't come out. I was going to just bypass the flasher but wasn't sure why they were there... I bought the truck 2nd hand, and the trailer signals have never worked. If it matters, its a 90 series.
Tonester