I don't have any information on your great grandparents but a good source that I have used is Familysearch.com.
Good thing is they have both church and civil records for Merida.
Bad thing is that they are not indexed so you have to search manually through the microfich they have up.
Most of the films have indexes for the volume you're looking at. Some are at the front and some are at the end of the book.
Best of luck.