Hey guys thanks for the info! I solved it today! I first removed the coating on the right side of the trunk and then removed the foam and saw the air pomp, but there was nothing wrong with the pomp.. So then i removed the rubber hose by the roof on the right side ( that was the spot where i hear the air sound) i saw that the yellow air cable was completely torn!

So what i first did was checking in my girlfriend's beauty case and found a small rubber hose to bind the torn peaces together and now it's working again! I think this is a weak spot because of opening the trunk door again and again.

I think the pomp was working for about 8 seconds because it had no good pressure.

Sorry for my long story.. Again thanks guys for the help!