Air Conditioning problem
2001 Mercury Marquis V8 Two Wheel Drive Automatic 114,00 miles

I only have heat, even when my A/C unit is on and the temperature is at 60. My mechanic says the door sensor is bad but he was able to give me A/C by either propping open the "door", or shutting it, I'm not sure which. All was good and I had A/C until my battery went dead. When the battery was repaired, I was back to heat. My mechanic says he can't get the door open again - or closed, whichever is right. Where do I find the door? I was told there may be something blocking the door and I would like to investigate.