Duff wiring or stuck switch somewhere, is it your air conditioning staying on, as in the little switch with the orange light, or is it your blowers are permanently on? If it's just the air con itself and not the blowers I would bet on a short in the wiring or a knackered switch, Give it a few good flicks to see if it goes off, easy to get on ebay or from a breakers if its the switch, a little harder to trace if its a wiring short.