Why is the trunk of Mercedes CL 500 opening halfway?

If the CL you are referencing is a 98 or 99 - which in Mercedes factory speak makes it a 140 chassis, the trunk was designed to be opened by remote control fob. There is a strong spring which will open the trunk completely once the lock is triggered, either manually on the trunk, or by the remote key fob.

If you mean the trunk is only opening half way, the spring can be broken, or it may not be fastened tightly on the hinge, but you should be able to easily open the trunk the rest of the way by hand.

If the "soft-closure" mechanism is not working property, it can be reset by taking both fuses out that are identified as to that system for 15 seconds, which will allow the computer to reset. It should begin working again unless the system has failed completely, and then there is a central Bosch system box that needs to be replaced.