Planning a trip in September to Trujillo and Caceres and am trying to get information on traveling by bus from Madrid to Trujillo.

Several books mention that this service is available but do not reference the bus company. I tried the great consolidated bus company link that Eddie found and couldn't find anything (of course, that could have been a user issue ).

Auto-Res is the bus company that goes to Cáceres and Trujillo.

A million thank you's Antonio. Should have checked this out earlier but couldn't find it. This actually also answers my question on how to get to Elvas from Extremadura!

Now, posting a related question:

Which auto-res station in Madrid goes to Trujillo and Cacares?

Cathy, Auto Res has its own station in Madrid at Fernández Shaw, 1, though the route to Lisbon (and a few others I think) stops also at the Estación Sur de Autobuses in Méndez Álvaro.

Antonio-
Which station would be closer to the airport?

Cathy, both stations as very close to each other. However, notice that only one or two routes stop at Estación Sur. They all depart from the main bus station owned and operated by Auto-Res at Fernández Shaw street (subway station: Conde de Casal).