Open your boot, and look on the top of the bumper bit that the boot closes onto for 5 (or 4) metal squares and a plastic push button. The whole unit is screwed on with two standard screws, un-screw them and pull the unit out.
There should be 5 plugs connected to the other side of the unit, check to see if any of the plastic has melted etc which could cause a dodgy connection to the wiper motor.
If you have the SX then its likley the heated rear window plug has melted everything