I have a 1986 Pontiac Grand Prix which I believe is haunted. A couple of weeks ago, after a long drive on a very hot day with the A/C on, I stopped and shut the car off. As I was getting out I noticed the A/C was still going full blast, even with the key in my hand. My father disconnected the wire on the blower motor under the hood so my battery wouldn't die. Since then, I have to hook up and disconnect the wire every time I use the car or I'd drop from the heat. I thought maybe you could give me an idea what the problem is before I bring it to the garage and possibly spend a lot of money.
The blower motor continuing to run with the ignition off is a common problem, but not a big one. It's typically caused by a bad relay. A relay is a heavy duty switch that is activated by an electric current from another lighter duty switch. So when you put your A/C fan speed on high, that little fan switch in the dash sent a small amount of current to the "blower relay" under the hood. This current activated the relay, which completed the circuit from the battery directly to the blower motor. This process avoids the electrical overload of the smaller switch when large amounts of current are required to power the motor at its maximum speed. Relays are also used in many other automotive applications for the same basic purpose.
In your case, after a period of time, the electrical contacts which complete the circuit inside the relay got stuck together. This, in essence, wired the blower motor directly to the battery permanently. And turning off the A/C or even the ignition switch wouldn't turn it off.