the entire mechanism seems to be integral to the wiper motor so probably needs a replacement motor but could be just a dodgy connection so try unplugging and plugging the wiring connector at the motor; fairly easy to get at; remove wipers then all the screws holding the plastic facing down this is in 2 parts and there are press in clips also; the screws at the side of the windscreen were rusted on mine but they are only screwed into plastic and came out with a drill bit set to vibrate and reverse; take the opportunity to replace the pollen filter. My daughter had turned the engine off with the wipers going so they were then frozen in the non-parking position; when she started the car and left it to warm up the motor burnt out; this is why on the continent you see the wipers pulled forward away from the screen when the car is left; I got a used motor from ebay for ...£30