The power wire supplies key on/off power to the GT tuner. This will allow the device to power on and off with key cycles. Install the power wire into a 10 to 15 AMP key power fuse. Find this fuse by probing with a test light if it has power when the key is on and no power when the key is off this is an ok fuse to use. (You may notice a black ground wire in the power wire, this will not be used! The OBDII block will be your ground for the key on/off power.) Do NOT use any fuses that are for safety equipment such as ABS, or airbags.