Fot the Mexican bus system, go to www.ticketbus.com.mx,
there is an English version, sort of - the Spanish version is not too difficult to follow.
From Corazol to Chetumal, and vice versa, George Morales will take you across the border - these drivers know all the border people and are a great help. His e-mail is [email protected]
His tel. number is 501-4222485, or was the last time I spoke to him last year.