Friends John and Jane have a blog, several blogs actually, and now quite a few books out there as well.
You can access all their stuff through this blog link
One of the things they've posted about, and have recently updated is about taking buses and colectivos here in Merida and the surrounds.
They have put together a very concise listing of bus stations and destinations, along with photos of the stations.
This is Must Have info, you should bookmark this for future reference.