Try searching on "San Carlos" Mexico
(be sure to include the country, as there are other towns named San Carlos in California and elsewhere) and other nearby location names. On a quick scan, I see several sites on the first page of search results for "San Carlos" Mexico
that appear to list vacation rentals. See if you can either get a free listing or purchase advertising on those sites.
Try pay-per-click (PPC) such as Google Adwords, Yahoo, etc.
Are there any travel agents who specialize in vacations in the San Carlos area? See if you can partner with them, perhaps get mentioned on their website, or get them to recommend your vacation rental to their customers who are planning to visit the town.
Are there any festivals or special events that will be taking place in or around San Carlos? See if you can purchase ad space on the festival or event website.
If you start to look around, you will likely come up with many other places you can promote your rental.