When I purchased my 1998 Land Rover Discovery 2 TD5 the tailgate lock would lock using the central locking remote, but would not unlock.  To open the tailgate I had to go inside the car and use the internal door handle.  I removed the tailgate inner panel which revealed that someone had already been there before, since the plastic weather seal had been cut around the lock.  The lock is held in place by 3 Torx headed screws which were removed.  The central locking electrical cable was disconnected.  Two linkages that attach to the external handle and the internal lock button were disconnected.  After sliding out the lock, a mechanical cable that connects to the internal handle was then released.

These photos show the lock as removed from the car, it was dirty to say the least.

I thought the dirt may have been causing the mechanism to jam so I cleaned it thoroughly using WD40 and my air compressor.

After cleaning I reconnected the central locking electrical multiplug and tested the lock on the car but the mechanism was clearly not working, so it was time to take the lock to pieces.

First of all I removed the tailgate external handle control arm.  This is released by unscrewing two Torx headed screws.

Hidden behind the other arm that can be seen in the photo there is a spring clip that secures a pin through the centre of the lock this clip was removed and the pin pulled out.

With a little pulling an prizing it was then possible to separate the two parts of the lock mechanism as can be seen in this photo.

There are 3 Torx headed screws and numerous clips around the edge that hold the two parts of the unit that contain the central locking motors.  These were released and the unit prized apart.

The motors are push fitted to the case and so can be simply prized out for examination.  I traced the circuit and worked out which pins on the multiplug connected to the motors and the microswitch.

The problem turned out to be dirt inside one of the motors.  I disassembled the faulty motor and cleaned the commutator and brushes with switch cleaner.  A 12v battery was used to test the motor which now ran as it should.

It was then just a matter of reassembling the lock which was quite tricky.  I also lubricated the mechanism with some white grease.  After refitting the lock to the tailgate it was tested and proved to be functioning correctly once again.