I just bought a 2006 S-type. A fuse blew that works the trunk And rear doors.I read the owner's manual....WOW there is a Key Hole under the trunk lid to the left of the trunk release button, just put the key in and turn clockwise, and the trunk opens.

 

X350 also has that key hole, do make sure you keep it lubricated and test it from time to time, lots of people do not and come the time they need to use it they find it is seized up.

 

Very good advice right there!
Those manual key slots never get used so when you need them they may not work!
Another big one is the drivers door key slot on the X351 XJ. It's hidden and never used as the manual key is hidden inside the key FOB.
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You're in the wrong forum for your car.
This forum is for the 2010 - 2019 XJ.
There is a different forum for the S-type.

 

The way we finally got ours to open is my three son-in-laws got there fingers under trunk lid while i pressed the open button and lifted up and it popped open. Thank goodness!