A few months back, I remember reading an article where aftermarket radio
manufacturers were complaining because car makers were making the radios
more and more specialized and they couldn't be replaced because it would
affect the operation of the car.

One car I thought they refered to was Volvo's.  They said that the radio
unit contains not only the radio but controls for other parts of the
car as well.  I think they meant that alot of the steering wheel controls
go back through the radio and would become useless.

Is it possible to replace the radio in a 1999 Volvo S80 (or any other
modern car) and retain all the functionality of the radio as well
as other car functions?

Or do radios do more that than just tune stations?