Dirt may settle in the bottom of the reservoir and block the pump. Getting at the pipes is difficulty. The front pipes have a heating wire built in so don’t be tempted to cut the pipe to gain access. When I had your problem I collided not clean the blockage by flushing out the bottle so had to remove the pump which is not hard. My local MB dealer quoted £127 for the job. Follow this process it only takes 20 minutes.
* Park car with front wheels on full right lock.
* Remove two 10mm nuts from the rear of the passenger side front wheel arch lining . The nuts are plastic so don’t rust and live in a small recess in the lining one near the bottom and one 3/4 of the way up.
* next step is to remove two plastic push in clips at the very base of the lining. Use a small screwdriver to release the centre pin with it pulled forward use plyers to remove the clip.
* carefully pull the lining towards the wheel staring from the base, it only needs to be moved enough to gain access. Gloves are recommended as it’s mucky.
* you should now see the pump at the bottom of the wing recess. It is removed by pulling it up, a large screwdriver may help to aid its release
* if successful the contents of the bottle are now escaping
* on the end of the pump will be a 2cm long rubber bung with filter holes in it. If it’s not stuck on the pump it’s still in the reservoir just carefully prize it’s away.
* wash the black rubber bung well pulling it a little to open the filter pores.
* replace the bung into the reservoir and then push the pump into place.
* top us reservoir with a little water to test that the bung is correctly back in place
* close the bonnet and test the washers. Note the washers will not work with the bonnet open another little known feature that’s not in the hand book
* if all is ok replace wheel arch lining the two clips and two nuts, top up the washer with water and screen wash.
Hope this helps.