Gabriel Gonzalez wrote:

> How do you determine the size of the series resistor???

  As large as possible, of course!  (In ohms)

  The larger is is, the greater the protection.  I realise that in a
harness checker, you really want a low resistance in order to
distinguish a resistive fault (though less likely in manufacture).

  A PIC seems to be an obvious choice *instead* of a 4051 as they are
generally held to be robust, have at least 8 available wires, are
inherently reversible (can act as source or sink or sensor) and are
intelligent, so can greatly simplify the circuit.
--
  Cheers,
        Paul B.