Rear Fog, wiring using existing factory harness

wilsonp

I think you have the wrong terminal highlighted in your connector diagram - you need terminal 5, but have highlighted terminal 3.

I believe you are correct, if you connected output 3 to input 5, the BCM would read the fog light switch as on and turn on the rear fog light.

The circuit shows diodes for the switches (except one) in each column to prevent cross-talk, and you would need a diode for the fog switch. Without a diode, when you had the fog switch on and another function on input 5 enabled, the BCM could read the wrong switch settings and activate a third device.

Of course, this assumes the firmware isn't different for cars without factory fog lamps and the BCM code still has the routine for enabling the fog lamp. Update: Since the BCM configuration indicates WITH is set for both FR and RR Fog Lamps, I think it is probably included.

Interesting design - it uses 5 output and 5 input pins to read a 25 switch matrix, but only needing one switch per function. This is the way old calculators read their keyboards (and new, cheap ones today), by scanning for closed switches. I would have expected a binary encoder to minimize BCM lines, but that would have required more wire lines to the combination switch and I guess software is cheaper than hardware in cars, like the old days.

Funny that it says reads 20 switches but actually has 21.

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