You don't say what model A Class you have. Is it one of the earlier W168 series up to 2005. Or the newer W169 version.
If you want now info there is an A Class owners club online. One of the members has his own website with loads of valuable A Class info. Including loads of pics.
Here is the link to the relevant page regarding the rear wipers.....
http://www.aclassinfo.co.uk/mypage.42.htm
I hope this helps?.
Tony, March 2009
Hi,i am not a mechanic but i do own a A Class Merc with the same problem !!! I haven`t had mine fixed yet but i do know the Tube has detached from the Jet and the water is now running into the Hatch.
You will find it dripping out of the bottom and onto the floor.