rear brake lights staying on

[stoichio]

Update. Made a little headway. Asking what changes w/key in acc, and w/key in engine run position. Checked voltages and grounds at Light control module (LCM). All looked normal with one exception. Found battery voltage at pin 47 with key in run position, engine off. This wire runs between the ABS/DSC Control module and LCM. Looking at wiring diagram it pertains to Brake Light Activation (Hill Descent Mode). Why voltage when the engine isn't even running? I'm not using cruise control. I removed the wire from connector to LCM. The brake lights are working/corresponding correctly to brake pedal position, on/off, no matter key position #1 or #2. Yea! Don't congratulate too soon though. A new dilemma popped up. With the engine running, the tach dropped to zero, and the engine temp gauge pegged at high temp. Engine was cold for sure because I just started it. I realize the wire at pin location 47 and this new dilemma may all be connected. Suggestions?