Not sure whether you mean the switch is hard to turn or the windscreen wipers are struggling when turned on.
If the former, probably easiest to just buy a new switch. If the latter I would suggest disconnecting the windscreen motor from the car and connecting the terminals on the motor directly to a charged car battery
with an appropriate fuse in-line and see how the motor operates. This will tell you if the problem is with the motor or the wiring on your car.
Poor wiring on the car is reasonably easy to fix, albeit probably time consuming. If it's the motor, I'm not familiar with it so would not be able to offer further suggestions. Although I'm sure it would be repairable.
PS. I have just uploaded a 603 owners manual in the
Resources section, although it won't help with this problem.