The connector on the back of Bonneville radio doesn't accomodate an add-on in-dash unit the way most other GM vehicles do. About the only way you can achieve what you're looking for (keeping the OEM radio) is to add the factory XM components. Contrary to what GM will tell you, this is possible but requires the addition of new wiring and the purchase of what may be some expensive components. The XM receiver is a separate module which mounts in the trunk, there is a roof mount antenna and various other parts. 1993 SLE and I have both done this. I not sure if any other members have. The lower centre tile in my signature shows the new XM receiver mounted in my trunk.
StingRay Bike w/cards in the spokes, Weeeeeee!