OK, let's define "flashers" Use a term we can know for certain, as you have SIGNALS and you have HAZARD or "4 way" flashers
So is it the turn signals or 4 way that do not work, or both?
First thing I'd do is go around, take all lenses off and clean the socket shells and bulb contacts, and the bulbs, or replace them.
Second is consider the flasher units. There are two, one for the hazards, and one for turn signals
Grounds is one of the no1 bigtime troubles. Make sure the sockets are grounded. Turn on the tail lights. Take a meter or test lamp and go to each socket. Ground one probe of the lamp / meter and make CERTAIN the lamp is grounded, then stab the other probe directly onto the socket shell. Any lamp indication or voltage shows the socket is not grounded.
Also, "these" troubles can be right in the bulbs. Make CERTAIN they are the correct bulbs. Bulbs can develop shorts internally that crosses the filaments together, which causes some interesting results.
