Usually, it means the flasher is getting weak or you do not have enough juice to run the flasher. I have had both battery was on it's way out as soon as I changed it problem went away. Buy a new flasher and replace what's in there for the signal lights.

Hazard has it's own flasher

 

Thanks OZ...much appreciated! The battery is only two years old...seems to be plenty of juice. I will change the flasher as you suggest.

 

Lets us know how it goes, it could also be a grounding issue or trailer light wires have rotted away and are no longer connected.

I should have asked all the signal lights work in the truck if you flash them manually ?

 

Hi Oz, I will keep you posted. I will check to see if the wires have worn as you suggest. I will also check out the manuel approach to. I know when the signal arrow stops flashing on the dash board the exterior lights aren't flashing either. If I try to mimic the flash by moving the switch arm on-off...well sometimes it works, but most times no. I am going to price out the switch tomorrow and get some instructions to swap it out. Thanks again for your help...much appreciated.