I`ve got a 1991 H reg, 200TDI Series 1 Discovery manual, which when the wipers are work, operate fine, but when trying to switch them off or park them(what ever the expression is) they stop in the postion where you switch them off and don`t ride down to the bottom of the windscreen, like other cars do, i`ve read in the small history the vehicle came with and it mention something about a windcreen sender unit(??), can this thing be quickly remedied or is it something that a more experienced person should attempt at?