Don’t understand why it matters, you want relay power connected to battery.
Alternator supplies the battery.
The trigger for the relay is what flows through the wiring harness, and only requires mA, not amps.
That’s the whole point, remove the large current drain through the factory harness and supply it directly from battery.
Ah ha! That's what a lot of us thought. But in these systems the alternator output does not connect direct to the battery.
The 'typical' wiring arrangement on these cars and trucks was for the alternator to supply the circuit feeds at a welded junction inside the car.
The battery feed joins the alternator feed at this junction.
The battery feed/charge wire is a dead end at the battery.
^This^ arrangement is pretty elegent as it only needs one fusible link, keeps the charge line use to minimum, etc.
It has inherent weakness when equipment needs to be run off the battery, either because its run when the engine is off, or because the amp loads are very high (winch).
It also means there is no noise damping of the current by the battery. That's not a problem with basic electromechanical or even older solid state regulation and ignition but just mentioning it as another reason why its not a design approach used in more recent decades.
Basic power scheme layout illustrated below. Details will vary with year, model, etc.