Why I send relay ground instead of 12v:

Sending ground to the relay means a shorter hot wire and maybe less chance of trouble/burned or shorted wire.....maybe less chance of voltage drop from a shorter wire.

If a long ground wire rubs through, it's only ground.....relay comes on, but no fire from shorting a ground wire.

It could also be a habit from running wires for GTE. their relays were always tripped with ground no matter what the relay was sending.

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Different subject:

Sensor wire in case cornering light was accidentally left on resulting in dead battery.
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When door opens, the light inside the rear mirror comes on for 30 seconds or whatever it is set for.
that will provide 12 v to the switch panel, which sends ground if accidentally left on and combined together the buzzer should go off.

2. Steering wheel to mirror is the shortest path for the sensor wire.
...far shorter and less complicated path than all the way under the hood.to fuse box or to cornering light,

and easier to hide the wire behind the interior trim than threading it through the maze,

like I did a few days ago with the switch ground wire.

I can also manually push the light for a test without opening the door.