i wanted to keep the Infinity 7 channel surround sound system that was in my 94 Chrysler LHS. the head unit is the same 1/5 DIN as the GM stuff, but brackets might need to be made and wires run all over the car- the system uses a rather powerful (500 watss total, i think) amp that is mounted in the trunk.
but it just sounded beautiful for a factory system.
of course, it did have a lot of speakers- 6X9's in the rear deck with dual voice coils (low and high), 6" round speakers in the front doors, 4" speakers in the dash, and a small center channel mounted in the middle of the dash.