Some intel from the web on this:
Brake and turn signal is the same wire.
Why does BCM require a rear turn lamp feedback signal? Found on a GM Diesel forum:
Found on this site: http://www.justanswer.com/chevy/8xu6v-chevrolet-volt-i-m-looking-tail-lights-wiring-schematic.html#1 Yellow/Brown Left Rear Turn Signal Lamp Supply Voltage
2 Light-Blue/Gray Left Tail Lamp Supply Voltage
3 White/Violet Rear Turn Lamp Feedback Signal
4 Black Ground
5–6 Not Used
1 Light-Green/Violet Right Rear Turn Signal Lamp Supply Voltage
2 White Right Tail Lamp Supply Voltage
3 White/Black Right Rear Turn Lamp Feedback Signal
4 Black Ground
5–6 Not Used
Exterior Lighting Systems Description and Operation
Park, Tail and License Lamps
The park lamps, tail lamps and license plate lights are turned ON when the headlamp switch is placed in the PARK LIGHT or LOW BEAM position or anytime the headlights are requested. When the BCM receives a request from the headlamp switch to turn ON the park lamps the BCM a sends out a PWM signal, which illuminates the park lamps, tail lamps and license plate lights.
Stop Lamps
The brake pedal position (BPP) sensor is used to sense the action of the driver application of the brake pedal. The BPP sensor provides an analog voltage signal that will increase as the brake pedal is applied. The body control module (BCM) provides a low reference signal and a 5-volt reference voltage to the BPP sensor. When the variable signal reaches a voltage threshold indicating the brakes have been applied, the BCM will apply battery voltage to the left and right stop lamp control circuits as well as the center high mounted stop lamp control circuit illuminating the left and right stop lamps and the center high mounted stop lamp.
Turn Signal Lamps
Ground is applied at all times to the turn signal/multi-function switch. The turn signal lamps may only be activated with the vehicle ON/OFF switch in the Service Mode or ON position. When the turn signal/multi-function switch is placed in either the TURN RIGHT or TURN LEFT position, ground is applied to the BCM through either the right turn or left turn signal switch signal circuit. The BCM then applies a pulsating voltage to the front and rear turn signal lamps through their respective voltage supply circuits. When a turn signal request is received by the BCM, a serial data message is sent to the instrument cluster requesting the respective turn signal indicator be pulsed ON and OFF.
Brake and turn signal is the same wire.
Why does BCM require a rear turn lamp feedback signal? Found on a GM Diesel forum:
2007+ GM trucks and 2008+ GM vans use the BCM to flash the turn signals instead of a flasher relay. The BCM checks for burned-out turn signal bulcs by measuring the load, and uses a "fast flash" to notify the driver that a turn signal is burnt out. GM (and presumably every other manufacturer) had to do this because of this government mandate:
"A standard production vehicle will have BCM software that checks for two bulbs functioning in each rear turn signal bulb circuit. If one or more bulbs are inoperative, the BCM will “fast flash” the Instrument Panel Cluster (IPC) turn signal indicator as required by Federal Motor Vehicle Safety Standard (FMVSS) 108 and Canada Motor Vehicle Safety Standard (CMVSS) 108." (from GM Upfitter BULLETIN # 81)
