the air dam is spring loaded. When the sunroof slides forward, the actual roof pushes down on the tabs, lowering the air dam. You kinda have to see how it works to understand it.

Its probably dry, and needs a good lube. Look very closely for any small broken pieces of plastic, but beware, sunroofs on the w124/126 are amazingly complex, and difficult to work on.

I had a similar issue years ago, and a bit if lithium grease resolved it.

Good luck, and post some pictures when the problem is resolved.

__________________
Eugene

10 E63 AMG
93 300te 4matic
07 BMW X3
14 Ford F-150 Fx2