2002 Gran Prix eletrical problems

I have a 2002 Gran Prix gt, it is having some major problems, I just recently had an ignition switch put in because there wasnt enough power to run the heater at the same time. Now though, when I turn the ac on, my car just shuts off but keeps power, I am getting very mad and confused why this is happening, any ideas? Oh and one time I went to start it and it ran for a second or so, shut off and the alarm went off. AND the speedometer and odometer dont work anymore either. Even the mechanics dont have a clue, thanks!