I have not posted anything on this site before but thought it might be helpful for some if I gave an account of this problem, as I couldn't find any other posts relating to this specific problem.
I have a 2006 X-Type estate which I have had since last August, it is in mint condition with everything working as it should. However over the last few weeks the alarm has been sounding in the the daytime and seems to only happen when the sun shines! - although not much sun at the moment it happens when the car is parked at work and is quite annoying for everyone.
I usually only single lock the car therefore not activating the internal volumetric alarm, but has a part of the diagnosis put blue tack over the internal sensors but the problem still exists.
Everything else works normally, so I called the local dealer who told me they would charge £130 to diagnose the problem - a bit steep bearing in mind they'll also be charging an unknown amount on top to fix it! They also said they could disable the alarm siren for the same cost (£130). The problem with this (which they didn't tell me) is that it does not stop the horn from sounding.
So as I'm not too bothered about have the alarm sounding anyway I set about disabling both the siren and the horn, the challenge was do this but keep the horn working on the steering wheel switch. Disabling the siren (backup sounder) is the easy bit - front wheel off, remove wheel arch lining at the front and unplug the siren -done!
The next bit was to stop the alarm from setting off the horn. With a bit of research and managing to download the official Jaguar X-Type electrical guide the solution was quite straight forward. The main unit that controls the alarm is the Generic Electronic Module (GEM) which is located in the driver footwell, the electical guide gives the circuit diagrams and the pin outs from the GEM. Basically the GEM takes an input from the steering wheel switch (horn) and has an output that controls the horns under the bonnet via a relay, because the GEM sits between the two it enables it to sound the horn should the alarm be activated.
The solution is to remove the SW switch input and horn output wires from the GEM and then join them together so the switch then operates the horn directly - like the old cars!
Having done this the car remains silent but the horn still works via the steering wheel - at last! (The hazard lights do flash though for up 30 secs, for me this is not a problem)
The pictures below show the backup sounder, GEM and the GEM pin outs.