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All A/C systems have some form of low-pressure shutoff switch to protect the compressor from fluid lockup. If your evaporator "freezes up" the compressor needs to shut off to keep from continually icing it which will cause high-pressure liquid to back up into the compressor on the high pressure side by evacuating the low-pressure side through the compressor running when it isn't needed (the super-cold evaporator will cool the air without refrigerant running through it while it thaws).
Locate the low-pressure shutoff switch. Figure out what you have to do to force the compressor off; in my old truck removing the connector leaving the switch circuit open forced the A/C off while jumpering the switch terminals with a wire forced the A/C on. If this is the case splice into the wire (or one of the wires) with a manual toggle switch you can mount under your dash.
The switch will be located on somewhere between where the evaporator tube passes through the firewall and the compressor (not in the run that includes the condensor, the other half of the circuit). It will probably be closer to the firewall rather than near the compressor.
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