how does it work? does it fill the main tank as it gets low or is there a selector switch to switch tanks that i would also need?

The two tanks work separately. If your Bronco was not equipped with dual tanks, or everything has been removed, the will be other parts to consider. Such as, a filler neck, fuel cap, fuel selector switch and plate, dash fuel switch so you can read both tanks and switch the fuel gage between them. You may also need fuel line, if that is not in place.