It's not just today...
...I have been working on the electronics for quite a while now, and I've got the light switches working, and also the wiper system.
Unfortunately, something's happened to the microcontroller so one of its inputs isn't working correctly (it's floating at 0.16V instead of switching with the wipers). This means I may have problems if I replace it - I can't see why it's happened, except I did have one incident where a wire got very hot, due to the wiper system deciding to both park and go at the same time.
I'll replace the PIC with a spare, but if this one goes wrong, I'll simplify it a lot.