You may be able to check if its the motor as follows
If poss pull back both the wiper arms clear of the screen (as if you were cleaning the screen)
Make sure that there is sufficient clearance to operate the wiper in this position. ie dont hit bonnet
If they work normally when off the screen under no load then odds on the motors had it hence wont work under load with blades on screen.
This will also prove combi switch is OK
Maurice
February 2011