This sounds like a break in the wiring to the self-park contact on the
wiper motor. I don't know exactly how the interval wipe is implemented
in the Saxo, but the traditional way is to have the wiper motor supplied
through a contact in the housing which switches off (opens) at a 'park'
position. If you have the main wiper switch on, this is bypassed and the
motor just keeps going - if you have switched off the main switch since
the last cycle, the motor keeps running through the 'park' contact until
it opens in the 'parked' position, then stops.

The interval-wipe uses a relay which bridges the 'park' contacts briefly
- just enough to get the motor moving so the contacts are made again -
then drops out for the interval - say 5 seconds. During that time the
wiper will complete one cycle, but by the time it gets back to the
'park' position, the relay has timed-out and there is no supply, so it
stops as if you had switched off the main switch. A few seconds later,
it all happens again for one more sweep.

If the wire to the 'park' contact is disconnected somewhere, none of the
self-parking or interval stuff will work, but the main switch will still
supply current to the motor directly whenever it's on - but of course
that stops immediately you switch it off, and so does the motor,
wherever it is at the time.

Hope this helps

Bill