roby02, the stereos in the update XJs are very easy to remove. Just put the gearstick in 1-2, pull the dash surround off (just give it a tug with your hands), there are two bolts, take them out, pull the radio straight out and disconnect the antenna and wiring harness. The stock unit is 135mm, I think there is about an inch or so behind it for the harness.
Here is a pic of the adapter I took for someone else a while ago: