The first thing I'd do is to jump the two wires attached to the Low Pressure Switch(or pressure switch) which is mounted to the Accumulator Tank
(also know as the Receiver-Dryer).
Purpose of the pressure switch:
The pressure switch senses internal pressures and can cycle the compressor off and back on based on minimum and maximum pressure limits. Pressure switches also turn on or off you condenser fan or Radiator Fan.
It is possible for the normal automotive A/C unit to leak up to 12 oz. of refrigerant per year. If by jumping the pressure switch, the A/C compressor clutch engages, the unit is most likely low of refrigerant, or the pressure switch is defective.
I have run across many defective low pressure switches.
In addition, the A/C relay is usually mounted in the engine bay, on the passenger side.