My money is on the rear wiper motor leaking internally. Was a common fail on B5 onwards, so good place to start. Strip out tailgate and rule that out. The copper pipe sheers and eventually kills the motor. If that looks okay, then work back. On the B5 it used to run through the top hinge, down the driver's side in the avant (behind the trim) then over to the passenger side footwell. You can access water reservoir by removing wheel arch liner.
The connections are just click together pipes, so not unusual during cold months for water to freeze and pop if drying to use washers.