Headlight relay system and Convertible top relay system recommendation

matthon said
.... does anyone know if there is a specific reason power is taken from the alternator, as opposed to the starter relay or battery?

This -

crackedback said
Which item powers the car with the engine running?

To answer this, you need to know the following:
Electricity flows from the highest voltage source.
The Alternator should supply power at 14.5 Volts, although it may drop closer to 13.5 Volts at idle.
The Battery will be around 12.5 Volts, although it may briefly float around 13 Volts when fully charged.

Power flow when the alternator is working and the battery is fully charged will look something like this:

more here: Understanding Charging Systems with Ammeter