Do I need to disconnect the battery before installation?

Yes. Disconnect the negative battery terminal before unplugging any electrical connectors. This prevents shorts and protects your vehicle's electrical system during the swap.

Will this work if my original headlight housing is cracked?

This assembly replaces the entire housing, so cracks in your old unit don't matter. You're getting a complete new unit, not a repair.

What bulb type does this use?

Refer to your owner's manual or the bulb specification label inside the assembly. The housing accepts standard Mazda 3 bulb fitments for your model year.