You still do not understand. I asked four questions because I am TRYING to troubleshoot this for you. But if you wont answer the questions, I cant help, I am NOT standing next to your car, only you are. I wanted responses like this:

1A) A noise comes from the trunk area that sounds like a "whirr" sound and the siren sends out a chirp for about 1-2 seconds....
1B) The turn signals flash twice.



So let's forget that, you are pretty much on your own (but you knew that already), my thoughts are in blue below.

It's when I lock the car with the FOB the siren starts sounding. I have no lights flashing more than the green turn signal indicators on the instrument. The doors and tailgate doesn't lock either.
If the green indicators are flashing, then the front and rear turn signals are flashing, as they are all wired together, the indicators cannot flash apart from the turn signals. This means the SKREEM is processing the FOB signal successfully, as the SKREEM flashes the turn signals. The CLP/SSM can turn the fog/driving lights and tail lights on, if they were lighting up, it would imply the CLP/SSM has gone insane because those sets of lights don't come on simply because you locked the car.

I think they did some modifications to this car to adapt it to the Swedish market. So only siren sounds even earlier when it worked. Before it got to never work it was during a 6 months period that it sometimes worked and sometimes not (to activate alarm and lock the car). That is hard to follow, but it sounds like the car has had issue with the locking-anti theft system for some time. This makes it sound like the CLP/SSM has intermittent issues that are getting worse.

The siren isn't the original, some standard aftermarket and some modifications of the cars electronics to have it to work. That is a hard sentence to read, I think you mean the replacement siren was put in and someone modded the electronics to make it work - which makes no sense, as there are aftermarket sirens the bolt in and work simply by connecting them to the wire from the CLP/SSM. Again, what mods? The alarm siren itself is a very simple module, it takes the command from the CLP./SSM and makes noise. That's it, that is all it does. I doubt any mod has been made, but I admit I don't really know for sure. If the car is modded, I don't know of anyone who can help without seeing the mods and "reverse engineering" what was done.

When I lock the car by the key it only lock the driver door, nothing more happens. Even erlier when the alarm worked it was still only the driver doors that locked when using the key in the door. But doesn't matter for me. This means the door lock cylinder wires (there are two, one for lock and one for unlock) are not connected to the SKREEM or the SKREEM is not processing those inputs correctly. It's starting to sound like your theory of some sort of modification has been done, might just be accurate. Is there any chance an after market remote start, keyless entry or anti-theft kit has been installed?

But I think the problem is going to be solved know when I found the liftgate sensor not giving correct signal as it should. I doubt that, but I admit I cant say for sure.

By the way, on a car that isn't modified in any way. Is it possible to lock the car with FOB or key in the door without activate the theft alarm, only locking the car ? And how to do that ? The anti-theft/alarm system is integrated in what they refer to as the 'Power locks", so no.

Based on what I have read, I'd tear the trim from the trunk until you get to the CLP/SSM. Unplug EVERY connector from the CLP/SSM and look for water damage, that is, corrosion, on the connector pins and sockets. It is common for water to find its way into the wheel well, then follow the wiring harness down to the CLP/SSM, where the water corrodes the pins in the connectors, then seeps into the CLP/SSM, destroying various components. VERY common. If this is the case, the system will do all sorts of weird stuff.

The fact that (as I commented before), the issues seem to have become worse implies, to me, water damage in the CLP/SSM, as water damage is progressive, as the corrosion 'grows' and contact is lost between wires and pins in the connectors over time.

IF this is the issue, then the fix is to replace the CLP/SSM and the last 2-3 feet of wiring harness and all connectors. You can get an CLP/SSM and wiring harness from a donor "parts car". I've had to replace at least three in other owner's cars over the last 5-6 years.

If the CLP/SSM is clean and dry, then I'd go looking for aftermarket modifications. I'd then remove them and restore the car to factory and go from there.