TPB,
I'd do a walk-around the rig and make sure both rear brake lights work when your helper mashed the pedal. Have them indicate a turn and watch the front signal lights...they should illuminate but won't flash.
If all this happens, my guess would be the same as yours, it's the flasher itself that is bad. Follow the wire loom down from the turn signal unit. You will probably find the flasher tucked up neatly under the dash above the steering column...didn't Beltfed just say that!
Stuart
