In a Jag, if you don't start replacing stuff it ain't going to fix itself! You gotta make it feel like it's being worked on!

 

I don't know if it helps but I have attached the electrical wiring diagram for the wiper system along with instructions for removing the steering column switches.

It may also be worth checking out this thread regarding cleaning the rain sensor:

Jaguar Forum.co.uk • View topic - Erratic auto-wipe function: a story with a happy ending!

Having said that, if the wipers don't work in slow and fast mode the sensor isn't likely to be the (only) problem. As other posters have said, the column switch does seem to fail occasionally.