Three words -- Body Control Module. I don't know a damned thing about Jaguars, but with GM vehicles in general they all have a Body Control Module installed. Anything that isn't directly related to the powertrain is controlled by the BCM (incidentally, the powertrain is controlled by the Powertrain Control Module). In many GM vehicles, the BCM can be communicated with via the radio; this is to set certain user options like how long the headlamps will remain illuminated after exiting the vehicle. In the event that something goes wrong with the BCM, the radio will lock because it gets put into an anti-theft state, and typically the car will not start. All because a single capacitor on a shitty little Motorola board got cooked, for example.
Then, even if you get a used BCM with the same option codes as the one you're replacing, the radio will remain in an anti-theft state because the thinking of the designers (I guess) was that people would start swapping BCMs just to steal radios -- dumb.
GM, of course, has a tool to reprogram BCMs, but even they say there's a 50/50 shot their programming will render the BCM unusable. From my limited research of the boards they use, it seems there is little if any CRC done in any shape or form, so it sounds like the board will happily write bad or invalid data to the PROM.
Again, I don't know how a Jaguar design works, but there are vehicles where the radio does indeed affect other parts of the vehicle, much to the dismay of owners and dealers alike.
