I'm not sure I know what a 91 Corsica even looks like, so I'm just guessing that the low pressure switch is plumbed into the drier and that the drier is mounted close to the firewall. If you trace the low pressure line (larger line) from the compressor it should lead to the drier. Without connecting gauges to the system, the best way to test the low pressure switch is to just replace it. Around $12.00 at local parts store. I have also been told by a competent source that you can "tap" on the switch and see if this will cause the compressor clutch to engage. If you know anyone in the A/C business, doesn't have to be automotive, commercial, residential any A/C person will do. They can connect gauges and tell you the problem in less than 60 seconds. If you happen to have a can of r134a refrigerant in your pocket, and it is low, he can solve your troubles in 60 seconds more. If you go to your local parts store and tell them you need the low pressure switch for your a/c system, they can show you were it's located on you vehicle. If you can get your hands to it, it's easy to replace, unplug wires, unscrew old, screw in new, plug in wires.