Change Search Functionality

I’d like to change the way the advanced search functions, currently if you put a [word] in the [keyword] field it doesn’t search any of the custom fields. I’d like to have it search through the [map-location] field and possibly one or two more.

I understand I could add a second text field just to search that box, but I don’t think that’s good usability, modern search engines and browsers have “trained” users to use one box.

I know I don’t want to start hacking into the core files, but I’m unsure how to do it using a child theme and hooks, if someone could nudge me in the right direction, it would be appreciated.