There are two version of the control unit. One is the control unit for the RDC alone, the other version contains Alarm System and RDC.
65757713340 control unit for alarm system/rdc - up to 03/2008
65757718057 control unit for alarm system/rdc - 03/2008 and above
If you bought your GS with alarm system you might have the possibility to activate the RDC.
You still need the transmitter 65752337407, or better two. I do not know if you need the tire sensors, and also not what needs to be done to activate the RDC. May be you can let us know later.
The control unit is located in the compartment below the rear seat.