First of all, you won't find any wiring that goes from the steering column to the radio for steering wheel controls.
These controls ultimately connect to the Dash Integration Module (DIM). The DIM determines which button is pressed (I won't elaborate here as to exactly how), and sends a signal to the radio over the Class 2 Serial Data Bus.
In order to connect steering wheel controls, you will need the correct Inflatable Restraint Steering Wheel Module Coil (as the connection is made via this part), the steering wheel controls themselves and the correct steering wheel. All the wiring from the DIM to the steering column will also have to exist. As I recall, you will also need to have your DIM correctly programmed (via a Tech 2 scan tool) to tell it the controls exist. There is nothing that needs to be done on the part of the radio itself. The HU is the same with or without controls. If it sees a message on the Class 2 Serial Data Bus it will process it accordingly.
Here is a schematic of the Steering Wheel Controls for a 2004 Bonneville:
Not sure where the discussion about RPO JL4 came from, but this really has nothing to do with steering wheel controls. If you want to know if the (pre)wiring exists, check for conductors at cavities C11 (Remote Radio Control Supply Voltage) and D8 (Steering Wheel Controls Signal) of C202. They should be purple and dark blue wires respectively.