If it was an engine warning there would be a code displayed on the dash along with it usually.
If you leave the seat without setting the parking brake there is an alarm for that.
And on some models in the past if you leave the truck with key on and not running without parking brake set the horn would sound.
What is the full model and serial?
About the 3 wires....
one is power, one is signal for ecm and one is neg
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