I have a Jaguar X300 3,2 '96 model with a jammed boot lock. On the instrumentpanel in the dashboard it indicates that the tailgate is open. I've tried to open with the button on the tailgate and on the dashboard without any luck. It seems like the problem has influenced the central locking somehow cause the doors won't lock when I set the gearhandle in "D" or using the key-fob. Locking the car from the front door works though. On top of this the key-lock on the boot is not working so I can't open the tail gate as it is now.

Have you checked to see if the actual wiring to the motor has power? If you are getting nothing at the button it may not be the motor causing the issue. The emergency release is a mechanical pull so it is seperate to the motor.
Have you checked the fuses? There is a fuse for the tailgate in the dashboard fuse panel. With the drivers door wide open the fuse box is behind a pull off cover on the flat end panel of the dashboard. You need to check fuse 19 labelled 'T/LID'. It will be a 15A fuse, (usually blue).
Try this website for a Diagram of the fuse panel.

https://fuseandrelay.com/kia/ceed.html

You might have to drop the rear seats and go into the boot area open it with a screw driver.

Locking/Unlocking the doors from the outside
This is done using the RENAULT Card; see the "RENAULT Card" information in Section 1.
In certain cases, the RENAULT card may not work:
- if the RENAULT card battery is weak, flat, etc.
- if equipment operating on the same frequency as the card (mobile phones, etc.) is used;
- vehicle located in a high electromagnetic radiation zone.
It is then possible:
- to use the key integrated into the card to unlock the front left-hand door;
- to lock each of the doors manually;
- to use the interior door locking/unlocking control (refer to the following pages).
Using the key integrated in the RENAULT card
Insert key 1 into the lock in the driver's door and lock or unlock. Locking the doors manually
Turn screw 2 with the door open (using the end of the key) and close the door.
This means that the doors are then locked from the outside.
The doors may then only be opened from the inside or by using the key in the front left-hand door. Interior locking/unlocking door control
Switch 3 controls the doors, boot and, depending on the vehicle, the fuel filler flap simultaneously.
If a door or the tailgate is open or not closed properly, the doors and tailgate lock/unlock quickly.
If you need to transport objects with the boot open, the other opening elements can still be locked: with the engine stopped, press switch 3 for more than five seconds to lock the other opening elements.
Locking the doors without the RENAULT card
For example, in the event of a discharged battery or the RENAULT card temporarily not working, etc.
With the engine switched off and an opening element (door or boot) open, press and hold switch 3 for more than five seconds.
When the door is closed, all the doors and the tailgate will be locked.
Unlocking the vehicle from the outside is only possible with the RENAULT card in the vehicle's access zone or using the key integrated in the RENAULT card.
Door and tailgate status indicator
With the ignition on, the warning light integrated in switch 3 informs you of the locking status of the opening elements:
- indicator light on, the doors and tailgate are locked,
- indicator light off, the doors and tailgate are unlocked.
When you lock the doors, the indicator light remains lit and then goes out.

The sensor is located and probably built into the boot latch. The sensor is good because the light goes off if you close it slowly. the latch is just worn out with that particular part. Replace the latch. It is not hard to replace if you have a few hand tools.

you may have a broken wire around the boot hinge. or a faulty boot latch switch, also check the switch on the boot lid.itself.also the plastic boot closing panel has not come adrift, has the boot switch will not operate if the panel is missing/adrift.

If there isn't an emergency "lock knackered" panel in the tailgate trim, you will probably have to force the trim panel off. These are usually only fastened by plastic pop studs, so you should be able to do it without damaging the panel, although you may need to get replacement studs.

When the tailgate is fully closed, is it actually properly latched down? Try pushing down firmly on the tailgate when it's closed just to be certain as it's possible that the latch is sitting half open at the moment.

either the wires in the rubber boot from the door to the door frame are damaged or their is a wiring plug off in the tailgate door itself or the lock has gone ,its a strip and play job to find the fault

I have same problems on 2003 model. considering addding remote keyless power locks to avoid costly repair.

You can use your ignition key to open the boot, it has a key hole on or under the boot chrome trim

The micro-switch is fitted inthe lock of the tailgate.Try opening the tailgate and manually close the lock and see if the boot light and the dash indicater light go off,if not,replace boot lock.If the lights go off,tailgate not closing properly,adjust the boot lock catch.Good luck