4 way flashers on 1998 Ford Escort zx2 with 104,000miles

My 4 way flashers(emergency flashers) isn't working. At first both the directionals and 4 way flasher stopped wokring. Replaced fuse, flasher, directional switch, flasher switch, traced wires etc. All work done by cerified ASE mechanic. Problem was not located. Mechanic stated by luck, unknown what he did or touched the directionals began to work, but 4 way flashers still not working, Anything you can suggest to do? Mechanic cannot located problem through hours or tracing wires and replacing all obvious parts. Thanks