Buy this connector for $50
and download this free software
You'll want to get the windows version if you want to modify any of the on-board modules. This will allow you to read the codes, reset and modify ALMOST ANY setting in your cars computers. Powerful and free for download.
At least you will know where the problem lies. (I'm guessing a corroded connection).
PS there's probably a way to turn off the system using this software. (Personally I don't know the memory location, but almost certain it can be turned off using ForScan).