The BCM (body control module) is a module that controls/monitors most electronic things in the truck. Usually when people refer to ECU, they are referring to the PCM (powertrain control module) which controls/monitors engine and transmission related information.
If your third brake light doesnt work, I would check to make sure its not leaking. There are handful of people who have had third brake light leaks causing electrical mayhem.
If your cluster is staying illuminated with the key out, the first things I would check:
-The ignition switch; stick your key in and gently wiggle/push/pull it. If the cluster flickers, you likely need and ignition switch and or tumbler.
-The shifter; shift from park to neutral and back to park several times (like 50 times). Does the 'P' illumiate in the cluster EVERY time? If not, it is possible your 'park detect' or 'brake shift interlock' is not functioning properly. Also listen carefully when you push the brake pedal, you should hear a faint click (brake shift interlock).