can someone tell me what functions the body control module controls
The body control module (BCM) controls all body functions like exterior lights, interior lights, chimes, windows, instrument cluster, theft deterrent, wipers and washer, remote key fob and remote start.
It may control some other functions according to options.
To tell you exactly I would need to know the correct year and engine size and model of Impala to help you better.
If there is a problem explain what is going on so I can help you diagnosis the problem.