Well, I had a bad case of slow wipers and had got as far as sourcing a replacement mechanism - I then found I couldn't get that trim piece off, no matter which way I pulled / lifted / slid the wretched thing. When it started to crack I gave up and decided I'd have to put the job off until I'd got a replacement ready to hand. But, as an interim solution, I took advantage of the slightly increased access to apply some spray grease to the linkages and the spindles. (NOT WD40!)
The end result of that was wipers which worked again - and a year on, still are. It might just be worth the try - I suspect that you may need to put the arms on to give some manual assistance to the mechanism to get them moving and help the grease work in.
Mike