It sounds like the PO installed a switch in order to bypass the Klima relay. I suspect that the Klima relay is defective so the PO just bypassed it. There are times (hot days) when the Klima relay gets overheated and shuts down the compressor. When this happens on my car I pull the Klima relay and wrap a cold wet cloth around it for a few minutes, reinstall it, and everything is fine. A new Klima relay is about $150, but you might get lucky and find a good one from a wrecked car. Your compressor is staying on all the time because the Klima relay has been bypassed by the switch.