The amplifier takes input from the pressure switch (prevents operation if pressure is too high or low), the tach signal (engine speed) ,the thermistor (evaporator temp), and A/C switch . If conditions are right and you command it on then, with an on-board relay, it will engage the clutch and switch on the A/C idle up solenoid.
You could rig up a relay to the A/C switch that could turn the compressor on, But if the evaporator gets too cold it won't switch it off before it freezes it up. Or if the there is not enough freon you will burn up the compressor in short order.
The amplifier circuit is pretty simple and someone with electronics knowlege could fix it or you could find one in a junk yard.