Audi: 2000 A4 Climate control only with lights on??

I have a 2000 A4 TDI which has developed a fault with the climate control.
If the headlights or side lights are on the climate control works perfectly.
If the lights are off it exibits trange behaviour. The temperature "-" light
is on all the time, the temperature "-" button does not work, the fan "+" &
"-" buttons don't work and the recirculation button does not work. Repeated
pressing of the temp "-" button sometimes lowers the temperature and
sometimes switches the air from "auto" to face vents!

Anyone seen anything like this?

Thanks,

Jim.