The file that contains network location info is /Library/Preferences/SystemConfiguration/preferences.plist. You can try deleting that & rebooting, although I would definitely back up. It looks like mine only contains networking info, but better safe than sorry.
This seems overkill, though. AFAIK there's nothing special about the "Automatic" location. It's just the one that's already there. Just make sure "Ask to join new networks" is checked for the Airport Preferences, and under the Advanced -> TCP tab you're getting an IP via DHCP or assigning one manually.