After cursing the rain sensor for over a year (delay was always too short or too long) I discovered that I don't have a rain sensor and therefore I had a fault with the intermittent wipers. As you might have guessed it turned out to be a dirty potentiometer track in the switch. Glory be the switch is easy enough to get out and is even screwed together. Unfortunately the wiper support is held onto it's shaft by one of those spring clips that are almost impossible to remove without breaking the shaft and I failed again. There was just enough shaft left to hold the clip so I was lucky. A quick squirt of contact cleaner and a dab with a piece of kitchen towel and I have useful delay settings again. If you want to check the pot it is on pin 3 of the switch and ground is pin 6. The readings I get are around 2K, 4k, 8K, 16K, 32K at the 5 positions.