It can be stretched, but it is not easy. I did mine on the bench when I had the wiper motor assembly out of the car. I changed the wiper motor, etc. from an earlier vintage 124 to my '95 car. The wiper assy. I got from the boneyard did not have the whole gasket (batwing?) on it so I used the old one and stretched it around the replacment assy.

It helps to have the batwing warm, to make it stretchier. I used a heat gun on mine to warm it up, (don't melt it!) but I guess you could put it in a bucket of HOT water also. Have some strong wooden sticks available for levering it around the assy.

BTW it's not that hard to remove the wiper assembly, once you're in there. And then you can easily change the interior air filters.

Rgds,
Chris W.
'95 E300D, 297K