How do you make a rubber buffer that is supposed to dampen a glass tailgate TIGHT again and, if necessary, also tight?

The black plastic buffer in the upper half of the picture is probably supposed to dampen the glass tailgate (Toyota Aygo, identical to Peugeot 107, identical to Citroen C1). In my case, these dampers, which are level with the top edge of the taillight, wobble both on the left and above the right taillight. Water…