Adding relay to a rear window defrost switch

[Pukar Risal]

Hi Eric,

I watched your video on ground side vs power side switching and it was super helpful. I was hoping you could help me with an issue I have after installing an aftermarket android stereo. So the unit comes with android screen and an integrated climate control unit. All the functions in the climate control unit works except for the rear window defroster. I was told that it was the canbus controlling the rear window defroster. I tried different cabus settings and it still did not work.

Long story short, I tried to trace the wire and the switch pin that triggers the rear window defroster. To my surprise there wasn’t a wire connecting the rear window defroster relay to the defroster switch in the new unit. I traced the pin that worked when the switch is on.

The voltage on the relay side (car harness) shows 12V meaning I would need a ground side switching to turn the rear window defroster But the voltage on the switch also shows 12v when I turn it on. meaning the rear window defroster would not turn on because the switch is also supplying positive volt. My plan is to use a relay to invert the voltage from the switch to negative/ground

My question is, if I am wiring for a ground side switch is it necessary to add a fuse in the circuit? Do you think adding a relay would work in this case.

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