Gen 4 BSM mirror indicator circuit access

As you may already know, there is a buzzer in the blind spot monitor system, but it’s used only for a rear cross traffic alert. Perhaps Toyota decided for human factors reasons not to use it for the blind spot monitor; I’d be annoyed by chimes for vehicles passing or being passed in adjacent lanes.

The Electrical Wiring Diagram (more info) shows that the BSM indicator LED in each side mirror is powered directly from the blind spot monitor sensor on the same side of the car, via wire-to-wire connectors IM1 (light green wire at terminal 39) and and JM1 (blue wire at terminal 39), which connect the floor wire harness to the right and left front door wire harnesses, respectively. I think you could get access by removing the cowl side trim boards and a few other panels, without having to remove the entire lower instrument panel.

The Repair Manual (in the Terminals of ECU topic) says that 2.5 to 7.5 V, relative to body ground, is provided at the sensor when the indicator is illuminated. There is no discussion of how much current can be drawn, but apparently it’s monitored, since the sensor is said to store a diagnostic trouble code (C1AB2 or C1AB3) if there is a short to ground. I’d hesitate to experiment; the list price for a sensor (88162-0W270) is $865.82.

As an alternative, I imagine you could get the same information—along with turn signal status, which would let you suppress or quiet the warnings when no lane change is imminent—by monitoring one of the car’s CAN buses, but you’d have to reverse engineer the message formats. (The openpilot project, which I can’t otherwise recommend, might be a place to start for that.)