There are two hard plastic lines that carry air from the tank in the trunk. The one with the red stripe closes the door, the one with the green stripe opens it.
I have a manual for a 123, and it says to remove the actuator, remove the two screws and the clip at the top of the rod. Then you rotate it and pull it out.
Your problem is almost certainly where you heard the hissing sound.
There are two types of actuators: the older type can be repaired and is larger. It has two white plastic caps on the top and two more on the bottom.
The newer type can'tr be repaired and has a couple of small hex bolts on the side with the plunger thingie that connects to the door latch.
The older type with the external diaphragms can be repaired. You also might find the leak in the rubber connectors that slide on and off the hard plastic yellow air tubes.
I imagine that your 126 is similar. Good luck.
claims to have repair diaphragms and new actuators for the newer type.