Audio Systems: 2001 suburban, onstar, existing system

Question
I want to upgrade my system, its non-bose with onstar, if I change my head unit will I retain my cassette player or lose that function? secondly how will it affect the rear audio controls?

Answer
Best thing to do when interfacing to an existing system (and not replacing it) will be to go to www.crutchfield.com and speak to one of their online reps or use their toll free phone line.
They can easily cover all aspects of this in a very short time.

JM