Search in the regular John Deere forum (this is the Lawn and Garden forum) for the term "radio", there are several threads detailing removal to install new radios/speakers. The 3x20 cab are that same I think so that will broaden your search. The seals are critical to get correct or the AC will not work correctly.
Have you thought abut buying a service manual? The CD's are under $100 and worth every penny.