I do not believe this is possible.
On S Type built between 1999 and 2001 they used a standard harness for all specification variants which meant this was possible to updgrade when ever requested.
On S Type built 2002 onwards - there was 2 levels of vehicle harness supplied when built.
These were dependent on customer order - originally specified requirements.
This mainly related to the telematic/infotainment systems.
So in this instance I do not believe this is possible.
The plugs will not be present, plus there would need to be major changes made to the vehicles software installation identity to allow it not only to work but to be recognised by the rest of the systems on the vehicle.