Your finding out why most mechanics charge by the hour for electrical problems instead of a flat rate. just keep pluging away at the problem. Another way of going about finding it would be get a helper to push brake pedal and watch light while you go around and lightly pull and shift wires around at the four corners lights hoping to to get the problem to stop. If you get lucky and you get the blinker light to go out you'll know the general area to find the problem. also i would consider unhooking the trailer light plug or making double sure all the splices are good Sometimes people dont do a good job when adding in the trailer plug