How do I fix the dash light on my 2001 Yreg Citroen Xsara Picasso ?

When I switch on my lights when it's raining or dark etc I can't see the speedo or info on the dash everything goes really dim. I tried pressing the contrast button but it didn't work. Any ideas would be appreciated.
Y Reg 2001 Citroen Xsara Picasso 2.0 HDi