Citroen ZX estate electrical fault - help please?

When I switch on the lights, the demisting button automatically comes on, and I can't switch it off. When the lights aren't on, I am able to switch the demisting button on and off and it illuminates as normal, although I am not sure if the rear window heater is working.

Another problem is that the number plate lights don't work and putting in two new bulbs didn't fix the problem.

Any ideas as to what the problem is? Thanks.