I was driving back from caching near Anaheim Lake and saw a plaque next to an office park. I decided to see what it was for, and learned that this location was once the site of Juan Pacífico Ontiveros's adobe residence. Ontiveros was the owner of Rancho San Juan Cajón de Santa Ana.
There are three townsites listed on the plaque. Enter those three townsites in order to get the coordinates. 3 words only, no conjuctions. Watch out for autocorrect! The final is within easy walking distance.
EDIT 1/12/2022: Some sleazebag made off with the plaque. I don't know if the county is planning to replace it, or if they even know that it is missing. Since the plaque is not there, a picture of the plaque is now in the image gallery. You can use the image to find the townsite names needed for the checker.
EDIT 1/30/2023: The metal plaque has been replaced with an engraved stone plaque that contains the same text as the metal one. Since the information is available on site again, I've removed the picture of the old plaque and you'll need to visit the posted coordinates to gather the information.
You can validate your puzzle solution with certitude.