Your problem seems a little different.
You are saying that when you switch off the car, the blower keeps working, hence draining the battery.
The problem I had was, when I used to switch OFF the car from ignition, the engine kept running, if the blower was ON. So in order to shut down the engine, I had to switch it OFF from ignition as well as from blower.
The A/C guy installed a small additional part with the relay, he called it electrofier or something like this.
If you are in Rawalpindi/Islamabad, you can go to that guy.
And if you are in some other city, I am giving you his number, he can tell you the exact details
Abu Bakar, Universal Car A/C: 0300-9192687