Air conditioning was working, now won't come on

I had someone trying to convert the cassette player in my car to a cd player. He was completely incompetent, and not only couldn't get the new stereo to work, but in the process managed to take out my interior lights, my trunk, and my air conditioning. I was able to get everything working with the exception of the air conditioning by switching out some blown fuses. The air conditioner still wont work, won't even come on. It was working perfect before. All of the fuses have been checked and work fine. One person stated that the fan in the motor blower had gone out. My questions are, how could he have blown out the fan putting in a new stereo? Secondly, would this cause the air conditioning to not come on at all or would it be an electrical issue?