Hi Mark,
         I've got the schematic here.On the "subtract replay line"there seems to be the following switches.Zero position switch on replay unit,open(slate+white)wire.......W relay normally closed switch(black+red)wire...,open switch on S relay(Blue +red).....motor switch 2C open(blue+red)......motor switch 1A open(red with yellow tracer)
        Loose wire or poor solder joint maybe,dirty or not enough overwipe on switch.
                                    Good luck
                                       Ian